What is O' supposed to mean?

Hello folks.
I've seen the term o' (o + apostrophe), but I can't figure out what it means.
I found it in the title of a song: Sweet Child O' Mine and in an ad which said: New Editions for the Good O' Fellows.
Could anybody help me with that? I guess it might be some abbreviation or short form for another word... Anyway I'm not aware of its real meaning and origin.

Thanks in advance.

O' = of (ex: lots o' luck; top o' the mornin')

"New Editions for the Good O' Fellows."

Are you sure the ad didn't say "good ol' fellows"? In this case, ol' means "old".

"Sweet Child O' Mine" = sweet child OF mine. When this phrase is said very fast, it can sound like "sweet child o'mine"....same as "top o' the mornin'" etc.

Thank you guys for your replies.
By the way, Eastie, the ad really says "New Editions for the Good O' Fellows."
I have it on a magazine advertising some English-learning material.
Now I just can't figure what they meant with that...

I think it would still mean "old".

Auld is another form of Old.

And don't forget "o'clock", originally "OF the clock".