Nearly everybody uses oh my god to state surprise or amazement. These days most people wouldn't think that this is offensive to believers. Oh my gosh is rarely or hardly ever used. Some British English speakers would say good gosh, but this is rarely used these days.

The TV programme 'Friends' had the expression 'Oh my god' as its catchphrase. The expression predates that TV show when it probably had a stronger meaning, but nowadays the meaning is watered down to just mean general surprise. 'Friends' was pretty big in the UK which I think is a factor why its usage has increased, especially among younger people. Some people overuse it a lot, as Su.Ki alluded to. You probably have noticed by now that most people in the UK aren't particularly religious, so saying 'oh my God' isn't a big deal at all. People normally say that. I don't really agree with what your colleague said, although perhaps she was speaking for herself. I wouldn't take it as some general rule, you can believe in God and still say 'Oh my God', it's not a big deal.

Gosh, golly, goodness (not godness) .... these are all softened forms of the word 'god', dating back many years, to a time when people would take offence at using the 'g' word inappropriately. These days, very few people are offended by this. Many people, especially young girls, exclaim 'Oh my god' (not good), three times in every sentence, without even realising they're saying it. This is just an expression of surprise, or simply an all-purpose response. It has nothing to do with religion. What your colleague told you isn't really accurate. If anything, the distinction is one of age rather than belief. The younger the person is, the more likely they are to use the now meaningless three syllable exclamation 'Oh my god' at every possible occasion. And yes, it's mainly girls.