You can use mIRC's /aop command. See /help /aop. Here's a few examples:
/aop nickname #channel
This will make you auto-op 'nickname' in #channel, but only if he uses that nickname.
/aop nickname
This will auto-op 'nickname' on any channel you're an op in.
/aop nickname #channel 3
This will add a nickname's *!*user@*.domain to your AOp list, meaning they can join with any nickname, but as long as their [email]user@host[/email] stays the same, they will be opped.
Regards,