Should be working:
on ^@*:notice:*:#:{ echo -a OpNotice $+($chr(91),$chan,$chr(44) $nick,$chr(93),$chr(58)) $1- | haltdef }
$+(a,b,c) is for binding square brackets, identifiers etc in the line returned, it will look like: OpNotice [#Chan, NickWhoSent]: Content of notice
1) You dont have to check for $nick being op, as only ops can send or receive op-notices
2) Using the @-prefix, the event will only be executed if you are op (if Im not, I can not receive Op-notices, anyway... ).
3) Using the ^-prefix is for halting mIRC's default text output, therefore I added "haltdef"
4) Unlike "normal" notices, Op-notices provide a $chan, I think knowing what chan was onoticed might be useful