Oban to Arbroath is a TGO challenge route....
It would go something like:
Oban - Taynuilt ( quiet road) - Loch Etive E shore - Glen Kinglass - Victoria Bridge - Bridge of Orchy - Beinn Dorain - Glen Lyon - Aberfeldy - Pitlochry - Kirkmichael - Glen Isla - Glen Prosen - Kirriemuir - Forfar - Arbroath. You could just reverse this, of course.
Another option would be to go down by Loch Rannoch - or up Loch rannoch, I suppose, bagging Schiehallion on the way...
You'd need to wild camp as the "indoor" accomodation is very widely spaced in some parts of the route.