The OM2n allows TTL flash metering using the OM dedicated flashes, the OM2 does not. I'd call that something serious and it is well worth holding out for the n version. There is also an OM2 spot/program which, as the name suggests has a spotmeter and program auto exposure mode (the 2 and 2n have appature priority auto (+ manual) only). Unlike the other two, which have very long battery lives, the spot/program is very battery hungry (as is the OM-4, but not the OM4Ti), although the spot function is useful enough that you might feel the extra battery costs worthwhile.
As Earl says, none of the OMs lack a DOF preview, which is operated by a small push button on the lenses.
It might be true that the OM-2 electronics are no longer repairable, but I have two, one of which I have had since 1979 and I have found them completely reliable in very heavy and often rough use.
I also have 2 OM4 bodies (one Ti the other not). IMHO they are worth every penny of the extra cost. Their exposure system in particular is still unsurpassed. Their only small weakness, is that the film wind can be a little stiff if the film is not loaded exactly right, so that they can be a little temperamental with power winders.