Can I legally audio record someone without them knowing if there are signs posted on site “on video surveillance”?

The signs are irrelevant in OK. OK is a one party consent jurisdiction. If you are part of the conversation, you can record your conversation without telling the other party in advance. You can not hide a recorder in someone's car or office or home or park bench, bathroom, etc. and record in secret any conversation where you are not a participant in that conversation. But recording your conversation with someone else is fair game.