Trying to combine Oban with Glencoe is just a little difficult.
What you could do is to get off the Fort William sleeper at Crianlarich, and catch the bus to the Isle of Skye from outside the station. That goes through Glencoe on the way to Fort William.
If you stay on the sleeper to Fort William you could then have the morning in Fort William, then the lunchtime bus to Oban. That would also apply if you had caught the bus through Glencoe to Fort William.
From Oban there are two possible island trips (apart from Mull)-
the island of Kerrera on the little ferry (that can be done one way, then dine at Kerrera Marine and take their water taxi back into the centre of Oban),. The ferry leaves from Ganavan Sands about two miles out of Oban- a nice walk. But there is also a twice a day bus, the #417 goes down there in connection with school start and end times.
It would be possible on arrival day to arrive from Fort William, check in to your hotel, then take the afternoon school bus down to the ferry. Do the south island circular walk, go to the marina at the north end of the island, eat dinner there then the water bus back to town. A memorable arrival day.
OR
the island of Lismore- great walking over there. A ferry runs from Oban several times a day. The other way to do Lismore is to catch the very early #005 bus or the later Fort William bus to Appin.
It is about a 45 minute walk down to Port Appin for the little passenger ferry over to the north end of Lismore. Then walk the length of the island, and then the vehicle ferry back to Oban.
To do Glencoe from Oban take the morning Fort William bound bus to Ballachulish, and connect into a Glasgow bound bus; the same in the evening on the way back to Oban.
News is awaited, in maybe 6 to 8 weeks time as to whether extra buses are to be run this year on the Fort William to Oban route. Vague hints are being dropped.
