Hmm - I think you need to consider two balances - that of bass to treble, and one of colour.
If you are not to end up with a classic ensemble structure (like a string quartet) then you should think how you can construct a balanced set.
Then you can play with the balance by having parts of the ensemble be quiet while other parts play.
I guess if you do not go for a classic structure, you will end up making a lot of your own arrangements. By having the right balance of treble to bass and a spectrum of instrumental colour, you will have a lot of fun with your arrangements.
OK - to go with the oboe - here are some suggestions.
1. A wind based ensemble
Oboe , Clarinet, bassoon with Trumpet, horn and Tuba (might get a bit heavy at the bass end so you could swap out Tuba for trombone or bassoon for another clarinet).
If you know someone who plays bass clarinet or is willing to do that (they are expensive though) then that could be a lot of fun.
2. Mixed bag
Oboe, violin, cello plus piano. The piano is a great 'filler', just resist the temptation to have it too much in the foreground - keep it back mostly (like the piano in a jazz band rhythm section).