Major Opposing Muscle Groups
Chest & Shoulders (Front and Back) Back & Soulder (Posterior/rear)

Biceps

Triceps

Abdominals

Erector Spinae


Quadriceps


Hamstrings
This is only a list of some of the major opposing muscle groups there are several other groups such as: forearm extensors/flexors, hip abductors/adductors and anterior tibialis/calves.  Also of note the back consists of many muscles that aid in different body movements, but in general the greater back region and posterior deltoid muscle groups oppose that of the chest, triceps and front & middle shoulder group.