DJ_Sol, thanks for your help.
I instead installed
mIRC in C:\Users\My user\AppData\Roaming\mIRC\ and then installed Omenserve. Worked like a charm.
I'm not sure I haven't tried your suggestion, but if I did it haven't worked, because this was the only way I could make mIRC + Omen work for me.
Thanks again for your suggestion, it gave me the idea to set mIRC install folder to AppData\Roaming\mIRC.