Other Mike is one of two friends who are both named Mike, but is the second of the two you have met. Hence the 'other'. It is not necessarily a lower status, but used to differentiate in a conversation between two Mikes. Can also be refered to as "O to the M".
'Hey Mike, are you coming to lunch?' 'Yah I'll be there in five.' Oh, ok, Other Mike just called and said he was coming too.'