Amber, if it were me, I would skip the night on Oban and spend at least 3 nights on Mull. I love Mull, possibly more than Skye, and would recommend you stay in Tobermory as opposed to Oban. Tobermory is charming and there is a lot you can see and do there. I stayed 4 nights in one of the harbor B&Bs and parked in one of the free car lots on the harbor. It's lovely to be able to walk down through the shops and restaurants that line the harbor. Bring your camera as it's very photogenic. You can visit the Isle of Mull Cheese Shop (Sgriob-ruadh Farm), which is about a mile from the harbor. There are self-guided tours to meet the farm animals and learn about traditional cheese making. There is also a cafe and a farm shop.
Also, there is a short walk from the harbor car park along the wooded hillside to Aros Park with woodland trails, a lochan and really nice waterfalls. A walk from the other end of Main Street leads along a coastal path to the Rubha Nan Gall Lighthouse. There are amazing views out over the Sound of Mull. And another short walk up the hill towards Baliscate leads to the Baliscate Standing Stones. Nearby is the site of the Baliscate Chapel, the subject of a BBC archaeology dig.
You also might want to stop at Isle of Mull Pottery, which is just beautiful. They sell some in the shops along the harbor but the main shop is in the town up above Main St. (you will pass it if you go to Calgary).
The Tobermory Bakery is wonderful—definitely stop here for one of their delectable pastries, take-away sandwiches and coffee or tea. Their Millionaire's Shortbread is incredible! There is a shop at the end of the harbor that is farthest from the ferry terminal that sells handmade chocolate with a tea room, and along the way are some very nice gift shops - I bought half my souvenirs in Mull (and I'm picky about stuff like that). Lots of nice restaurants, too. There is also a catch-and-release aquarium that is interesting, too.
And don't miss Calgary Beach, which is about a 30+ minute drive—probably more like 40—from Tobermory. It's very narrow and the single track roads run through the mountains with some switchbacks, so you will need to go slow through much of it. It's well worth a visit to see the beautiful beach. On the way back from Calgary, you can stop at Glengorm Cafe (about 3-4 miles from Tobermory) and have some lunch or pastries at the lovely cafe there. There are some beautiful coastal views as well.
And of course, there is the Isle of Iona, the birthplace of Christianity in Scotland and filled with so much history. It makes for a wonderful day trip from Tobermory. You can drive to Fionnphort, then take the 10 minute ferry over to Iona, see the ruins and the history there, have some lunch, walk on the beach. It's an amazing day.
Can you tell how much I loved Mull? :-)