Use /var instead of /set
/help /var

Also, change \"notice\" into \".notice\" (. = silent)
And the inc %i 1 can be made inc %i.

The code should look more like this:

alias onotice {
  var %c $nick($chan,0), %i = 1
  while (%i <= %c) {
    if ($nick($chan,%i) isop $chan) .notice $nick($chan,%i) [@] $1-
    inc %i
  }
}

Good work though!