Can the Isle of Oban be reached in one day if leaving Edinburgh's Waverly station mid-day?
(Remember there is a bus and ferry in the picture besides changing trains.)
Oban isn't an island, so do you mean the Isles of Mull & Iona? Schedule shows a train departing Waverly at 11:15 & arriving at Oban at 15:27 Next train doesn't depart until 17:15. It's almost an hour ferry ride to Mull and another hour bus ride to the Iona ferry..... doesn't look promising.
Spending the night in Oban and catching an early morning ferry would work better.
But Oban's on the mainland. I remember going to a (very rainy) highland games there. Meanwhile, trains from Edinburgh leave at 1115 and 1715, taking about 4 and a quarter hours with a change in Glasgow. Arrive Oban 1527 and 2126. If you had to go beyond, and couldn't leave before noon I'd say no way.