Oban’s a bit tricky, there IS good stuff there, for sure, and I’ve been away from more or less ten years so am sure I’m a bit out of touch, but the only good trails I’m still aware of are in Barcaldine Forest, specifically Sutherland’s Grove.
Your best bet for trails would be to speak to someone from the North Argyll Cycle Club (website but also on facebook). Some of those guys are actively involved in trail building and what I’ve seen of the stuff they’ve built is really excellent.
In truth, if the weather is decent, I’d be more tempted to take the road bikes and ride from Oban – Taynuilt – Dalavich – Kilmelford – Oban, but beware the punishing hills!