I think that the 28 was only offered with an inboard as standard, first (1979) the OMC ZEPHYR 15hp 2-stroke gas sail-drive( diesel optional) and later (1984) a 10hp Universal Diesel inboard (conventional shaft + prop). I think that larger Diesels were also offered.
That is not to say that one or two might have left the factory with an outboard bracket instead of an inboard, O'Day seems to have been open to "special orders"!
Personally, I would much rather have an inboard on a boat that large, first due to the aesthetics (outboard would look "stuck-on" and out-of-place hanging out in mid-air on a bracket), second due to the problem of keeping even an extra-longshaft (25" shaft) outboard's prop submerged in any kind of sea (just when you might REALLY need the auxiliary!) and third the inboard would represent better weight balance, It is important to keep as much weight out of the ends of a boat to avoid "hobbyhorsing" (rocking fore and aft) on a beat to windward. Finally, an inboard can support a larger alternator for recharging batteries.