$解:(2)②如圖3所示,當(dāng)OC在OB左側(cè)時(shí)$
$∵∠MOC與∠BOC互余$
$∴∠BOC=90°-∠MOC$
$∵OM平分∠AOC$
$∴∠AOC=2∠MOC$
$∵∠AOB=120°$
$∴∠BOD=180°-∠AOB=60°$
$∴∠COD=∠BOD-∠BOC=∠MOC-30°$
$∵∠COD+∠AOC=180°$
$∴2∠MOC+∠MOC-30°=180°$
$∴∠MOC=70°$
$∴α=∠COD=40°$
$如圖4所示,當(dāng)OC在OB右側(cè)時(shí)$
$∵∠MOC與∠BOC互余$
$∴∠BOM=∠BOC+∠MOC=90°$
$∴∠AOM=180°-∠BOD-∠BOM=30°$
$∵OM平分∠AOC$
$∴∠AOC=2∠AOM=60°$
$∴α=∠COD=180°-∠AOC=120°$
$綜上所述,存在α=40°或α=120°使得∠MOC與∠BOC互余$