Is over-under the same as the Thai clinch?

No. Over-under is a wrestler's body lock. Thai is head control for knees.

How do I get out of over-under?

There is a deny / evasion window. Drill it in Practice Mode. A panic mash is a trip.

Only long enough to break or land a short shot. Do not pose in a wrestler's lock.

One arm over, one under. This is the wrestler's lock — head position, angle to the cage, then the trip or the throw.

How the over-under clinch works in UFC 6: underhooks, trips, throws, and the break window.