Here's a copy of the OK Trespassing law section that describes what you must do to post your property, look for what's required in your state and follow it to the letter.
1. Exhibiting signs to read as follows: "PROPERTY RESTRICTED"; "POSTED ‑ KEEP OUT"; "KEEP OUT"; "NO TRESPASSING"; or similar signs which are displayed. Property that is fenced or not fenced must have such signs placed conspicuously and at all places where entry to the property is normally expected; or
2. Placing purple paint marks on posts on the property, provided that the marks are:
a. vertical lines at least one (1) inch in width and at least eight (8) inches in length,
b. placed so that the bottom of the mark is not less than three (3) feet from the ground or more than five (5) feet from the ground, and
c. placed at locations that are readily visible to any person approaching the property and no more than one thousand (1,000) feet apart on land other than forest, and one hundred (100) feet apart on forest land.
B. Beginning July 1, 2005, when a landowner uses the purple posting to identify no trespassing, such marks must be accompanied by signs placed conspicuously and at all places where entry to the property is normally expected, explaining that the purple stripe means no trespassing, property restricted, or similar explanation of what the purple stripe indicates. Property that is fenced or not fenced and using the purple paint marks must have such signs placed conspicuously and at all places where entry to the property is normally expected. On and after July 1, 2006, no sign shall be required to explain the purple posting.