When my wife goes on a long bike ride she finds that her right oblique hurts, while her left does not. Does anyone know a possible cause of this, like improper form or saddle fit?
Asymmetric pain usually means either some sort trauma-based injury (e.g. banging your knee) or muscle imbalance. Without more detail on your wife's habits it's difficult to say which, but I can give you a general idea of how to isolate it.
If it is a trauma-based injury, the pain will typically be localized to a specific area (for example, the dorsal plane of the oblique), and will be mostly triggered by a specific motions (such as biking). A persistent trauma-based injury usually requires specific attention. Deep tissue massage (such as A.R.T.) will break up inflexible scar tissue and allow gentle rehabilitating exercises to build it back up.
If it is a muscle imbalance, the pain will be more generalized (the sufferer is more likely to say 'my hip hurts' as opposed to 'it hurts right here'), and will mostly be triggered by generalized stress. Fatigue, a variety of exertion activities, etc. Muscle imbalances are extremely common and the solution is basically to strengthen the weaker muscle, usually by doing some sort of focused strengthening exercise that will work out both instances of the muscle. One side will feel pretty exerted and the other will feel fairly fresh, but that's how eliminating imbalances works.