obbligato

adj to be included in a performance （演奏或演唱中）不可缺的, 必需的.

oafish, oak, oak-apple, oaken, OAP, oar, oarfish, oarlock, oarsman, OAS, oasis, oast, oasthouse, oat, oatcake, oath, oatmeal, oats, OAU, ob, obbr.(=Music TV), obduracy, obdurate, OBE, obedience, obedient, obediently, obeisance, obelisk, obese, obesity, obey, obfuscate, obfuscation, obiter dictum, obituary, object, object glass, object lens, object lesson, object to,