Eilean Donan from Oban

I'd say not. Tricky by bus unless you can pick up the Citylink service from Glasgow/Fort William to Portree/Uig (number 915).
It would be a 3.5 hour drive by car and really only the outside is worth seeing. The inside is largely reconstructed in the 19th century and it can be quite claustrophobic in the narrow corridors. I'd suggest looking for an activity closer to Oban.

Eilean Donan is the iconic Scottish castle that appears on all the tins of shortbread etc. The outside is stunning but like Skyegirl says, the inside is quite disappointing. It is a long way - allow 3 hours each way to drive. By bus it requires three changes of bus and is going to take 5-6 hours one way.

Personnaly I wouldn't even think about attempting this.

Oban is delightful, and Eileen Donan Castle very underwhelming inside. My husband's cousin really wanted to tour Eileen Donan as we were leaving Skye, it was tiny and crowded inside, totally refurbished about 100 years ago, so everything is relatively new.

I'd definitely skip Eilean Donan for all the reasons mentioned by others. We were there a couple of weeks ago and it was a big nothing. If you don't want to visit more islands (we loved Mull, Iona and Staffa), you could drive to the Glencoe and Ben Nevis area. Much closer than the castle and some interesting history and walks.

The tour of Dunoille Castle on north edge of Oban was good value when I did it couple of years back.