∵∠BOD=∠OAB+∠OBA
=(∠ABC+∠BAC)/2
=(180°-∠ACB)/2
=90°-∠ACB/2
=90°-∠OCB
∴△OGC为直角三角形
∵∠GOC=90°-∠OCB
∴∠BOD=∠COG