Hello David,
Your situation is too common...if we go by the "book" according to TDA #38: the OOT is binding if the action is backed-up and both skipped players (A & B) check.
I'd like to back-up to TDA #37. Players must act in turn! That should be enough to warrant a penalty. TDA #37B also states that Players should wait for clear bet amounts before acting!
So to answer your question (by the book) If A Bets, or A checks and B bets...the OOT can withdraw his all-in...

Penalties would be encouraged to prevent further occurrences.
Now...if you want my opinion

, TDA #38 is not necessary. I'd like to see the OOT held to at least his verbal declaration no matter what the skipped players do. Why should Players A & B be put in a situation that: #1 Either forces them to check or does it allow the skipped player to bet the exact amount of the OOT? I could never get a direct answer to that question. Action changing?...Hmm...don't even know if I understand that completely.

I know what I would do...I'd force the OOT to his action no matter what!
Okay...so here's the recap: Verbal is binding...Players must act in turn...Players must wait for clear bet amounts before acting...So how in the hell does this guy get off with withdrawing anything? I don't like it and they won't get away with it in my card room. What makes it even worse is the OOT could even raise in a situation where he did not go all-in but he bet a specific amount. Example: Player's A & B are skipped and Player C bets 500. The action is backed-up to the proper bettor and he bets 1000...because the action is different, the OOT can call, fold, or raise.

I say he can fold and surrender his 500 OOT, or he can call with 500 more...that's it, he will not have a raise option!