om1 & 3 are mechanicals, they could run at all shutter speeds without battery, but no metering is possible...since the battery is actually meant for the meters.
the centre average metering in the om1 could be a bit slow to respond sometime, but when it respond, it is very accurate.
om1 is fm2 equivalent less the LED feature and 1/4000s speed, it's slightly lighter, more compact and comes with a manual mirror prefire.
om1 uses the 1.35 mercury battery which is hard to find. some managed substitute it with the silver oxide 1.5v with or without adapter. myself use a 1.5v battery with an adapter. the battery last for a year min, even if you left the meter on in the dry box for weeks...i think it has the auto swtich off when in no light conditions.
om2 and 4 have electronic shutters. not sure abt the om4, but om2 only runs at B and 1/60s speed without battery.
the om2sp, om3's, and om4's have both spot and centre average mereting.
om2sp drains battery easy when in AE mode, otherwise manual and spot metering seldom gives problem.
imho, one of best feature about the OM is not only the compactness but also location of the shutter speed dial, which is located on the camera mount and this make setting shutter speed and f stops a lot more faster, esp on the manuals. all om bodies have no AF feature...forget about om if you have poor eye sights or cannot do MF.
btw, myself am om user...i got most my system out of the ebay.com, which is a lot more cheaper than if i were to get it from local 2nd dealers. i own a om1md and a om2sp, plus some 28 ~ 300mm primes and zooms. the 50/f1.4 and 100/f2.8 are my all time favourite.
this is a very informative site on the OM system of camera...
http://olympus.dementia.org/eSIF/om-sif.htm