The on OWNER event only works on now-extinct IRCX servers, IIRC. Or maybe that was the "isowner" comparison...

From versions.txt:

23/09/99 - mIRC v5.61
2.Improved support for IRCX servers:

Added property $nick().owner, isowner if comparison, and "q"
property to $nick(#,N,aohvrq,aohvrq) where "q" is for owners.

Added on OWNER/DEOWNER events, work the same as on OP/DEOP,
and $mode().owner/deowner properties.