oral

a.口头的，口语的；口(部)的

optimal, optimism, optimist, optimistic, optimistically, optimization, optimize, optimum, option, optional, optometrist, opulence, opulent, opus, or, or else, or rather, or so, oracle, oracular, orally, orang-utan, orange, orange squash, orange-blossom, orange-coloured, orangeade, orate, oration, orator, oratorical, oratorio, oratory, orb, orbit, orbital, orch, orchard, orchestra, orchestra pit,