The screws are the same on many of the Oberheim keyboards of that era - they are a black flat-head machine screw, and the head rests in a nylon grommet with a steel cover (not sure what you call those technically). I don’t know the specs of the screws, but we have an OB-Xa in the shop right now, and I’ll try to get some more info on that and post again later.
I’m guessing there is some sort of flexing issue going on with the keybed, so maybe it is not resting straight when it is screwed in. You might be able to fix that by doing some shimming in certain places (using a flat washer as a shim on the mounting screws) - but you’d need to figure out where to put those shims to cure the problem. Another solution might be to shim the keys that are not playing - here’s a DIY guide for doing this, but with a different style of key:
Since your keys will be different from those shown, just focus on the fourth photo - I’m pretty sure that will solve your issue.