Is it OK to call someone a fool?

Yes, if they are atheists.

The fool hath said in his heart, There is no God. Psalm 14:1, 53:1

Or if they trust their own hearts.

He that trusteth in his own heart is a fool. Proverbs 28:26

Jesus often called people fools. (So it must be OK.)

Ye fools and blind. Matthew 23:17, 19
Ye fools. Luke 11:40

O fools, and slow of heart to believe. Luke 24:25

And so did Paul.

Their foolish heart was darkened. Romans 1:21-22
Thou fool. 1 Corinthians 15:36

O foolish Galatians. Galatians 3:1

You're in danger of going to hell if you do.

Whosoever shall say, Thou fool, shall be in danger of hell fire. Matthew 5:22