NFHS Out of Bounds?
 

Ohio State game play... ball is fumbled near sidline... defender steps out of bounds accidently with left foot - dives in bounds to recover loose ball... illegal participation?

Key word in your play is "accidentally." Since this is a Team B player, it would have to be ruled that he went OOB intentionally to be IP. (9-6-2)

NCAA no longer has Illegal Participation anyway. But I am almost positive that this has to be intentional for it to be a foul on B in NCAA. A cannot ever go out even accidentally without violating the rules to my understanding. I am also not talking about being forced out by a block or some other action.

Peace

Correct, there is no such thing as illegal participation in NCAA rules.

Under NCAA rules, assuming that the Team A player who goes OOB is not forced OOB via contact with an opponent:
During a free kick or scrimmage kick, they may not return inbounds. (5 yard penalty)
During a scrimmage down, a Team A player loses eligibility by going OOB. If he returns inbounds, he remains ineligible to catch a forward pass unless/until that pass is touched by a team B player or an official.

The only restriction on Team B players with regards to OOB is that they cannot be OOB at the snap.