You can't go wrong with a T20. Turn the calculator panel round on the back so it says "for automatic use with OM2" and let the TTL flash metering take care of the rest.
Word of caution though: the Olympus manuals and literature from the time make such a big deal about the freedom of being able to just fire away at any distance or any aperture and the TTL flash metering will always take care of it. This is true to an extent, but there are limitations. If you shoot close-up with the lens wide open, the shots still risk being over exposed on anything but really slow film. At the other extreme, expecting it to put out enough light to illuminate something 15 feet away with the lens at F/8 or smaller is also likely to lead to disappointment unless you use fast film. As always, the key phrase is 'use common sense'.
Thank you for the awesome tip! So far it has been fine using that manual flash with the distance/iso/f stop calculator at its back, but I would love some TTL auto.