Cut ends off of the lemon, then cut lemon in half. Chop half of the lemon into small pieces, peel included; squeeze other lemon half and save juice.
Combine olives with 2 tablespoon oil, oregano, chopped lemon, and lemon juice.
Heat remaining 1 tablespoon olive oil in a pan over medium-high heat. Saute bell peppers and onion in the hot oil until onion is limp and peppers are still a little crunchy, about 5 minutes. Mix hot mixture into lemon mixture. Add salt and pepper. Serve immediately.