Well, as far as I know, most of crackers use combolists. If you want to crack OG, you need high quality combolists with og usernames and working passwords for them and this is either not easy to get or not cheap. If you want to crack a specified OG such as for example "cracker" then you would have to create a combolist containing "cracker" as username and multiple passwords for it, but I don't think it will be easy to bruteforce the password unless the owner put something easy as a password. This is what I mean as the combolist:
"cracker:password1
cracker:password2"
etc...