Re- fishing
Just wander round the harbour area, and the North pier - There are usually several companies who do on the spot bookings. One, Argyll Sea Tours, I see has a website and now does on line bookings.
If you want hiking, hop on the little Cal Mac ferry to Lismore where there is great hiking, about a 30 minute voyage.
That leaves from the slipway in the Calmac car park, not the main terminal for the big boats.
Or walk out of town or take the very infrequent bus about two miles south to Gallanach, where there is another little Cal Mac ferry to the Island of Kerrera. Your hotel will give directions and be able to tell you about bike hire. That one is a five minute crossing, and the South end of the Isle is a great circular walk, complete with ruined castle. Hopefully the little tea room survived Covid.
I'm sure you'd be able to hire ordinary cycles in Oban to take over to Lismore, Kerrera would definitely need to be mountain bikes, better walked.