What variants are out there?
OA-96 by Olympic?
I saw one that had the buffer tube assembly still on, but shortened. It also seemed to have the A2 style carry handle. Didn't get a good look at it......
What's the effective range with a 2x or 4x scope?
There are only a few,
-OA-96 is a postban version of the OA-93
-Professional Ordinance Carbon 15 pistol (postban)
-Rocky Mountian Arms Patriot pistol (preban)
Those are the only ones I am aware of. muzzle velocity and accuracy go out the window with AR pistols.
The OA-96 is no longer being produce. Your only option from OA is the OA-98. Professional Ordnance has just filed Chapter 11. I called them and they stated that is just a corporate reorganization, but usually most companies file Chapter 13 soon after filling Chapter 11 because they still can not get they're crap in order.
Accuracy for most AR pistols is fine at 25 yards and under. Also remember this is a pistol and not a rifle so I would not try to rely on shots past 25 yards. The only benefit is that under 25 yards most rounds still have enough velocity to penetrate a Level IIIa vest and below. If you plan on using a AR pistol I would only recommend the heavier .223 rounds (75gr +) because they fragment at a much lower speed or a round like the Winchester 64 gr Power Point Plus which relies more on expansion then fragmentation.
I have both a OA-98 and a Professional Ordnance 15 Type 97 pistol(2002 model). Both function fine and both are very accurate at 15 yards and under, which is the range I would only use them for. The OA-98 functions without problems using a standards USGI 30rd mags. The Carbon-15 pistol only functions without problems using 20rd colt magazines, and is very picky on 30rds USGI mags. I only use the Winchester 64gr PP+ rounds in my pistols.
I prefer my .223 rifles over my .223 pistols, but the pistols are more fun at the range.
I hope this helps!