I don't think that /onotice sends op notices as @#channel. It seems to send individual regular notices to people who mIRC sees as ops on that channel.
/!onotice #channel testing onotice
-> wildtime.beyondirc.net NOTICE ChanServ,user01,user02 :testing onotice
/!notice @#channel testing @notice
-> wildtime.beyondirc.net NOTICE @#channel :testing onotice
-genius_at_work
it does if the server indicates it's supported (if i recall right it sends WALLCHOPS or STATUSMSG= with @ in raw05)