They are 250cc. I don't think there was any connection between Ossa and Bultaco. I have read that the people behind Montesa were engineers who had started with Bultaco.

Ossa used the same basic engine in different forms to power trials bikes (the M.A.Rs in the clip), Enduros, Motocrossers, Road bikes and, probably the highest tuned version of the engine, a quite successful 250 GP bike.

The M.A.R.s are tuned to have torque right from an idle up. They have a pretty good snap to them in their powerband as well, but nothing like the Motorcross (Stiletto) version, or the Road bike version (Wildfire) which have far less bottom end with lots of midrange and top end.

I recall a friend getting the use of one of the 250cc Wildfire Road bikes for a day while his new 69 Triumph 650 Bonneville was in for servicing.
He said the little Wildfire was considerably quicker than his Bonny!