Rinse chicken, pat dry. Place chicken breast up in foil-lined shallow roasting pan. In mortar, place garlic, salt, oregano and black pepper, mash them well together. Mix garlic mixture with butter and lemon juice. With your fingers, carefully separate skin from breast, massage 1/2 of mixture gently between breast and skin, then spread the remaining mixture evenly over the whole chicken. Tuck wings under and tie legs together. The chicken will taste better if marinated overnight.
Place pan in center of rack and cook for 55 minutes at 350° F, basting occasionally. Cook uncovered at 400° F for 5-7 minutes.