Verses with the word certainty in the Old Testament (4 verses):

Joshua 23:13
Know for a certainty that the LORD your God will no more drive out any of these nations from before you; but they shall be snares and traps unto you, and scourges in your sides, and thorns in your eyes, until ye perish from off this good land which the LORD your God hath given you.

1 Samuel 23:23
See therefore, and take knowledge of all the lurking places where he hideth himself, and come ye again to me with the certainty, and I will go with you: and it shall come to pass, if he be in the land, that I will search him out throughout all the thousands of Judah.

Proverbs 22:21
That I might make thee know the certainty of the words of truth; that thou mightest answer the words of truth to them that send unto thee?

Daniel 2:8
The king answered and said, I know of certainty that ye would gain the time, because ye see the thing is gone from me.