when to use "go" or "o" as honorific pre

[dudnaito]

it seems almost random. is there something i'm missing.

ごちゅもん

お客様

[Belton]

The rule of thumb is "go" with words of Chinese origin. (essentially those using on readings) and "o" with words of Japanese origin.
There are exceptions but at the moment I can't think of any I'd have to look them up.

[Chronitsch]

i had a similar question

here is the link to my topic
http://www.japanesepod101.com/forum/viewtopic.php?p=27998#27998
with some things that might help you ^^