Basically everything Apizur said was spot on. I would like to add that lifting heavy weights (especially in the arms/chest) is probably actually counter productive and will actually decrease KO punching power. More muscle uses more oxygen as well which will in adversely affect conditioning if you are planning on competing. Instead of lifting heavy weights, you should be focusing on strengthening your core with body weight exercises, increasing body control and balance.
Body control and performing fast/relaxed/precise technique is what punching power is based upon. If you MUST lift weights, stick to multi-muscle group excercises such as squats/deadlifts, but with low weight and high reps. THIS IS IMPORTANT, do not do ANY bicep or tricep excercises, they add nothing to punching power and the extra muscle mass will slow your punches down, cause more muscle tension and use more oxygen. Punching power comes from your base of your legs and hips and body rotation.