October in Oklahoma.
So long as you don't show up at the time of special events (State Fair, a county fair, fall festivals, Pelican Festival (end of September at Grand Lake), horse race running dates or somebody's marathon race, the commercial RV parks should not need reservations, COE parks should be mostly empty during the week (still fill some on weekends) and state parks don't take reservations.
Some facilities, COE particularly, will start scaling back what campgrounds remain open, as their volunteer workers start leaving for winter destinations. The one I use most closes about 90% of the spaces on October 31st. Then the other lower cost CGs (like state parks and tribal parks) will fill up with the full timers who were living in the COE parks and hopping space to space every 14 days. We saw the impact of this last year when the sequester closed all the Federal campgrounds earlier than these people usually head south.
I personally tend to make reservations for a week at a time att my favorite COE, even off season, because there are particular campsites I like at different times of the year. But that I can do this also shows me just how unused these facilities become after the children are back in school and families don't have time to go camping.
