What do people usually mean by saying "oh! freak"? I looked up "freak" in the dictionary. The answer I got is "freak" is used for a person who is obsessed about something (fitness, looks, etc).
Please note that this answer is about, and as a result contains, swear words. Parental discretion is advised.
They're using the word "freak" as a euphemism, a sort of pseudo-swear. In polite conversation, it's generally not acceptable to use profanity. To get around this, there are many substitutes that people use which are somewhat similar in sound but are considered acceptable.
For example, instead of using the word "fuck", some people say "frik", "frig", or as you have heard, "freak". A replacement for "damn" is "darn". "Shit" may become "shucks", and "heck" is used for "hell".
Many religious people use "gosh" in place of "God", although this is an example of a different definition of profane.