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.

Thanks to everyone for your input. It is much appreciated.

Remember, everyone, that Eilean Donan is not actually a castle. It's a 1930s-era tourist attraction that resembles a real castle, externally at least.

It's certainly pretty, though. It's probably worth the small fee to gain access to the tiny island upon which it sits, if you're traveling in the area.

If you don't want to pay but do want a good photo, then drive past the castle heading towards Kyle of Lochalsh. You will cross a bridge. Turn immediately left into the community centre car park (which also has free toilets). You can walk down to the edge of the water and get a lovely photo of the castle across the water, with the bonus of no tour buses in your shot.

Remember, everyone, that Eilean Donan is not actually a castle. It's a 1930s-era tourist attraction that resembles a real castle, externally at least.

That is a very heavy over simplification of history. It was a real castle, which was destroyed in the Jacobite uprisings (in at least it's 4th iteration as a Castle). It was then reconstructed to the original floor plan (in so far as could be determined from plans drawn up in the early 18th century) between 1919 and 1932, at least partly as a war memorial to the Clan MacRae- there is a major Clan memorial in the grounds. It was not actually opened to the public until 1955. After the rebuild it was a family home, and is still owned (as a Charitable Trust) by the family.

The iconic war poem In Flanders Fields was written by John McCrae- a clan member.

If you really, really wanted to do it I think it would be a train at 0920 to Tyndrum or Crianlarich then a through bus to the Castle, arriving at 1536- but you just miss a connection on the way back.

From Oban then Duart Castle on Mull is a far better alternative- walk or taxi from the ferry at Craignure. That Castle is the seat of the Clan MacLean, but it too is a 20th century rebuild after almost 200 years of disuse (see the History page on their website for details of why that was).

https://www.eileandonancastle.com/

Another alternative is the ruined Gylen Castle on the Isle of Kerrera in Oban Bay, and another is Kilchurn Castle on it's island by Loch Awe (you can go up to it, but not into it currently due to Conservation work)- also accessed by boat from Loch Awe station when open. Kilchurn is a castle everyone drives straight past (or you see glancingly from the train).

Dunstaffnage Castle and Chapel is even easier to reach- a citybus ride from Oban on West Coast Motors various services, then short walk.

Close by is the totally different Ocean Explorer Centre.

If it's not too hot and you like some hiking, a travel vlogger I enjoy watching, Steve Marsh from Scotland, vlogged a trip to Kerrera, an island near to Oban. The island has Gylen Castle, and it's partially restored according to a Kerrera info page: https://isleofkerrera.org/see-and-do

Ah, I see that I ditto'd isn31c with Gylen Castle. Something else in the Oban area that I have on my bucket list is the Hollow Mountain of Cruachan Power Station, https://www.visitcruachan.co.uk/

Cruachan is on my bucket list as well- I went to the Visitor Centre once, but on a day when tours inside the Mountain weren't happening. Cruachan even has it's own summer only request [flag stop] railway station. The buses from Oban to Glasgow/Edinburgh (seasonally) also stop there.

Another non castle place close to Oban which is well worth exploring is the Bonawe Iron Furnace- the most complete charcoal fuelled iron furnace in the UK, which even made cannonballs for the Napoleonic Wars. A very interesting place. Use Taynuilt Railway Station, or the Glasgow (and, seasonally, Edinburgh) buses as well as a spasmodic local bus service.

If you go down to Kilmartin (on the 23 or 423 bus) you also have the ruined Carnaserrie Castle to visit, also https://www.kilmartin.org/

Kilmartin is one of the most important Neolithic and Bronze Age sites in Scotland.