And it came to pass
[1, 3, 5, 8, 10, 13, 15, 18(2), 19, 20, 25, 29]

(1) "Behold, it came to pass that I, Omni, being commanded by my father, Jarom, that I should write somewhat upon these plates, to preserve our genealogy."
The Book of Omni gets off to a great start for a book in the Book of Mormon with the words "Behold, it came to pass" but it goes downhill quickly from there. The purpose of Omni's little book is the same as the purpose of his father's book (the Book of Jarom) -- to preserve their genealogy. Exciting stuff.

The Book of Omni is the last of the "small plates of Nephi," all of which were written in first person. The "large plates of Nephi" (Words of Mormon through Moroni) are third person accounts found and collected by Mormon and passed on to his son Moroni -- who 1400 years later, after being transformed into an angel, delievered the plates (large and small) to Joseph Smith.)

(2-3) It turns out that Omni's little book wasn't written by Omni, at least most of it wasn't. Omni only wrote the first three verses, from which we learn the following things: Omni fought a lot with Lamanites, he was a wicked man, and he had a son named Amaron, who received the plates from Omni when he died.

(2) "I fought much with the sword to preserve my people, the Nephites, from falling into the hands of their enemies, the Lamanites. But behold, I of myself am a wicked man."

(3) "And I had kept these plates according to the commandments of my fathers; and I conferred them upon my son Amaron. And I make an end."

(4-8) Amaron was a bit more prolific than his father, writing five verses. From them we learn that after 320 years the more wicked part of the Nephites were destroyed, but the Lord saved the righteous ones. Then Amaron passed the plates to his brother Chemish.

(4) "And now I, Amaron, write the things whatsoever I write, which are few, in the book of my father."

(5) "Behold, it came to pass that three hundred and twenty years had passed away, and the more wicked part of the Nephites were destroyed."

(7) "Nevertheless, he did spare the righteous that they should not perish, but did deliver them out of the hands of their enemies."

(8) "And it came to pass that I did deliver the plates unto my brother Chemish."

(9) "Now I, Chemish, write what few things I write, in the same book with my brother; for behold, I saw the last which he wrote, that he wrote it with his own hand; and he wrote it in the day that he delivered them unto me. And after this manner we keep the records, for it is according to the commandments of our fathers. And I make an end."
Chemish wrote a few things in a single verse in the same book, from which we learn that his brother Amaron wrote what he wrote with his own hand (not somebody else's) and he wrote his five verses in a single day, after the manner that the Nephites kept there records, according to the commandments of their fathers. And with that, Chemish made an end.

(10-11) After Chemish made an end, his son Abinadom took over. Abinadom wrote two verses with three beholds, saying that he killed many Lamanites with his own sword. But he didn't know of any new revelations, "wherefore, that which is sufficient is written."

(10) "Behold, I, Abinadom, am the son of Chemish. Behold, it came to pass that I saw much war and contention between my people, the Nephites, and the Lamanites; and I, with my own sword, have taken the lives of many of the Lamanites in the defence of my brethren."

(11) "And behold, the record of this people is engraven upon plates which is had by the kings, according to the generations; and I know of no revelation save that which has been written, neither prophecy; wherefore, that which is sufficient is written. And I make an end."

(12-13) After Abinadom's end, his son Amaleki took over. He spoke somewhat concerning Mosiah, the king of Zarahemla, which was a new land that the Nephites discovered while fleeing from the evil Lamanites.
"Behold, I am Amaleki, the son of Abinadom. Behold, I will speak unto you somewhat concerning Mosiah, who was made king over the land of Zarahemla; for behold, he being warned of the Lord that he should flee out of the land of Nephi ... into the wilderness ... until they came down into the land which is called the land of Zarahemla."

Exceedingly (14, 17)

(14) "And they discovered a people, who were called the people of Zarahemla. Now, there was great rejoicing among the people of Zarahemla; and also Zarahemla did rejoice exceedingly, because the Lord had sent the people of Mosiah with the plates of brass which contained the record of the Jews."
When the Nephites arrived in Zarahemla, they found the land already inhabited by the people of Zarahemla, who rejoiced exceedingly when they saw the Nephites because they had the plates of brass with the genealogies of the Jews.

(15-16) Now you might be wondering where these mysterious people of Zarahemla came from. It turns out that they were also long lost Israelites That's right! They were seafaring Jews that, with God's help, sailed over "the great waters" from Israel after it was attacked by Babylon in 586 BCE. (The people of Zarahemla are called "Mulekites" later in the Book of Mormon. See Helaman 6:10)

(15) "Behold, it came to pass that Mosiah discovered that the people of Zarahemla came out from Jerusalem at the time that Zedekiah, king of Judah, was carried away captive into Babylon."

(16) "And they journeyed in the wilderness, and were brought by the hand of the Lord across the great waters, into the land where Mosiah discovered them; and they had dwelt there from that time forth."

(17) After arriving in the New World, the people of Zarahemla (aka the Mulekites) became exceedingly numerous. But from time to time they had wars and whatnot. And because they didn't write stuff on brass plates, they became atheists and their language (Reformed Egyptian) became corrupted since. So the people of Zarahemla and the people of Mosiah couldn't understand one another.
"And at the time that Mosiah discovered them, they had become exceedingly numerous. Nevertheless, they had had many wars and serious contentions, and had fallen by the sword from time to time; and their language had become corrupted; and they had brought no records with them; and they denied the being of their Creator; and Mosiah, nor the people of Mosiah, could understand them."

(18) But these problems were easily solved. Mosiah taught the people of Zarahamla how to speak in Mosiah's language (Reformed Egyptian -- like all indigenous Americans speak), so they Mosiah could write down their genealogies too. (Although they didn't get to write them on the plates of Nephi.)
"But it came to pass that Mosiah caused that they should be taught in his language. And it came to pass that after they were taught in the language of Mosiah, Zarahemla gave a genealogy of his fathers, according to his memory; and they are written, but not in these plates."

(19) After that, it came to pass that the people of Zarhemla and of Mosiah united together and made Mosiah their king.
"And it came to pass that the people of Zarahemla, and of Mosiah, did unite together; and Mosiah was appointed to be their king."

(20-21) Then someone found this big rock with engravings on it. Mosiah, with the help of God, interpreted the engravings and found that they were written by a guy named Coriantumr, who lived with the people of Zarahemla for the space of nine moons. ("Nine moons" is "Indian talk" for nine months.)

(20) "And it came to pass in the days of Mosiah, there was a large stone brought unto him with engravings on it; and he did interpret the engravings by the gift and power of God."

(21) "And they gave an account of one Coriantumr, and the slain of his people. And Coriantumr was discovered by the people of Zarahemla; and he dwelt with them for the space of nine moons."

(22) Okay, so where did this Coriantumr fellow come from? Well, Israel, of course, silly! All the indigenous people in the Western Hemisphere came from Israel at one time or another. The Nephites sailed over in 600 BCE, the people of Zarahemla (aka the Mulekites) did likewise in 586 BCE, as did Corantumr's people (aka the Jaredites) at the time of the Tower of Babel. (You can read more all about the Jaredites in the Book of Ether.)
It also spake a few words concerning his fathers. And his first parents came out from the tower, at the time the Lord confounded the language of the people; and the severity of the Lord fell upon them according to his judgments, which are just; and their bones lay scattered in the land northward.

(23-25a) After Amaleki finished telling stories about the Mulekites and Jaredites, he began to be old and having no seed, he passed the plates to King Benjamin, who succeeded King Mosiah, and who drove the Lamanites out of the land of Zarahemla.

(23) "Behold, I, Amaleki, was born in the days of Mosiah; and I have lived to see his death; and Benjamin, his son, reigneth in his stead."

(24) "And behold, I have seen, in the days of king Benjamin, a serious war and much bloodshed between the Nephites and the Lamanites. But behold, the Nephites did obtain much advantage over them; yea, insomuch that king Benjamin did drive them out of the land of Zarahemla."

(25a) "And it came to pass that I began to be old; and, having no seed, and knowing king Benjamin to be a just man before the Lord, wherefore, I shall deliver up these plates unto him."

(25b-26) King Benjamin told everyone to believe in prophesying, revelations, speaking in tongues (which, according to the Bible, no one did until the day of Pentecost), and getting saved though Christ (who wouldn't be born for another couple couple centuries).

(25b) "Exhorting all men to come unto God, the Holy One of Israel, and believe in prophesying, and in revelations, and in the ministering of angels, and in the gift of speaking with tongues."

(26) "And now, my beloved brethren, I would that ye should come unto Christ ... and as the Lord l liveth ye will be saved."

(26) But he that shall endure unto the end, the same shall be saved. -- Matthew 24:13

(27-30a) Before Amaleki makes an end of his speaking, he tells us about a certain number of Nephites who leave Zarahelma and to return to the land of Nephi. They were led by a stiffnecked guy that got all of them killed, save fifty. After that, it came to pass that there was another attempted migration to the land of Nephi, with Amaleki's brother among them, and no one has heard from them since.

(27) "And now I would speak somewhat concerning a certain number who went up into the wilderness to return to the land of Nephi"

(28) "And their leader being a strong and mighty man, and a stiffnecked man, wherefore he caused a contention among them; and they were all slain, save fifty, in the wilderness, and they returned again to the land of Zarahemla."

(29) "And it came to pass that they also took others to a considerable number, and took their journey again into the wilderness."

(30a) "And I, Amaleki, had a brother, who also went with them; and I have not since known concerning them."