Olympic skeet is similar to American skeet, with a few exceptions.
OLympic skeet is 25 targets/round, like American, but there are two more pairs of doubles, and fewer singles.
Olympic is shot with 12 ga only, however max shot load is 7/8 oz, the same as American 20 ga.
In American skeet, the gun may be mounted before calling for the target. In Olympic, the gun butt must be touching the hip bone when calling for the target.
American targets are thrown immediately on the call. Olympic targets are thrown with an unknown delay up to three seconds after the target is called for.
Olympic targets are thrown faster, approx 60 mph vs 50 mph for American.
Olympic targets are slightly larger.
Olympic skeet is more difficult. 150/150 will almost certainly win the Gold in Olympic.
400/400 against the best American skeet shooters will probably just get you in a shoot off with several others.