All OMs have DOF preview. The button to actuate is on the lens mount, lower right quadrant as you are holding the camera to shoot.
Just about everything you need to know about OMs is in the OM eSIF, at http://olympus.dementia.org/eSIF/
If I were to choose between an OM2(n) and OM4, I would get the OM4(T). The spot metering is worth the price of admission, and the brighter screens are a bonus.
If anyone can locate a 35UC for me ...
OM1, OM1n, OM2, OM2n, lenses, bellows, Varimagnifinder, (2x) 35SP, (2a) XA, 35RC, etc. I'm a sick person.