When you're getting to know me, Oz works find.
If you're a sub and it's more comfortable, Sir is acceptable.
I'm used to Sir from the SCA as well. So it is less meaningful to me as a sign of submission.

When I'm involved with you more intimately, online or itf, you would address me as Milord.

It's similar conceptually to Lord of the Manor. Yeah, I'm a medieval.

Occassionally I will refer to myself as 'your master' and there are 'ceremonial' times you might address me as Master. Usually if I am giving you something... or taking something I own.