travel between Oban and Inverness

Driving on the A82/A821 from Inverness to Oban will take you along the north shore of Loch Ness and through some great scenery According to Google allow a minimum of three hours. The road is one lane in each direction and you are likely to encounter tourist coaches, animals on the road, cyclists etc. so caution at all times. Oh, and let us not forget the Scottish weather! From what I was able to find on Google trains and bus both take 6-8 hours and involve changes. To get an idea of the road look at Google Maps or Google Earth. Surfaces are good but roads are narrow compared to what you may be used to.

Bus certainly isn't 6 to 8 hours, don't know where Google gets that from.
There is an hourly bus in the summer, two hourly in winter Inverness to Fort William, which takes 2 hours 5 minutes Then 4 buses a day in summer, 2 in winter Fort William to Oban, which take 90 minutes.
The worst case in winter is the 0910 from Inverness, with a 1 hour 15 minute connection at Fort William, arrives Oban at 1403.
That gives a ferry connection to Iona if staying overnight on the island.
The winter afternoon connection is Inverness depart 1510, Oban arrive 1923.
That morning connection in summer is an hour faster- 1010 from Inverness.

The bigger issue would be Aigas, near Beauly. There are buses to there but it is a very thin service. By public transport that would probably be pre booked taxi from Beauly station.