The Norwegian Film Institute sums it up extremely well:
Oh My God! is a humorous observation of children´s interpretations and experiences of sexuality. The film also looks back through grown-up eyes at the reality of being part of the in-crowd” and the lengths to which one is prepared to go to become a respected member. But first and foremost Oh my God! is a film about the orgasm.It's about three 11 year old girls exploring their sexuality, in particular their idea of orgasms.
It's been described as courageous, taboo-breaking and provocative. It was certainly funny!