Okla. Admin. Code § 325:25-1-28
The manner of selecting post positions of horses shall be determined by the Stewards. The selection shall be by lot and shall be made by one of the Stewards or their designee and a horseman, in public, at the close of entries. If the number of entries to any race is in excess of the number of horses which may, because of track limitations, be permitted to start in any one (1) race, the race may be split; or four (4) horses not drawing into the field may be placed on an "also eligible" list to be drawn into the race by preference and lot if any of the first numbers are scratched. Any horse which has drawn a post position in a race and enters again on the next day entries are taken, shall be termed an "in-today" horse; and if the race overfills, the horse shall be given no consideration whatsoever, except in stakes races. A horse on the "also eligible" list shall not be considered as an "in-today" horse until s/he has actually been given a post position.
Okla. Admin. Code § 325:25-1-28