Sorry, but the OM-1, OM-2 and especially the OM-2sp are not in the same league as the OM-3 and 4. The lower numbered were consumer models, whereas the 3 and 4 were for the professional market, thus manufactured to completely different standards. When it comes to the OM-2sp in particular, it was one of the most unreliable of the lot. Ask John Hermanson of zuiko.com (he is simply THE best OM repairman there is) if in doubt.Even better if you find a good OM-2sp
The OM-4T(i) does not have the earlier plain 4 battery problems. In addition, they added much improved seals for weather proofing, plus fast flash synch but only when using the F280 flash unit and then only at very short distances...to be honest, not very useful practically speaking.
The well known (and excellent) spot metering apart, even in non-spot mode, the OM-4 center weighted metering is a stellar performer. Nikon did not match it until the advent of 3D matrix metering - and some even argue it still couldn't match the OM metering system. Personally, because the standard metering is already very good, I find that I don't use the spot metering very often at all. But sure, when you have wildly varying contrasts in a scene, the spot metering is God sent.
The biggest downside of the OM system, of late, is that due to the 4/3 crowd and adapters, prime Zuiko glass has risen in price considerably. Luckily, I bought my kit(s) long time ago..