“OML” is slang text for “Oh my Lord”. It serves the same purpose in text and in speech as the famous “OMG” that has become ubiquitous on teen girl t-shirts across the United States (and it’s usually one of only three or four different designs available without being incredibly frilly and girly or overly black and bleak). While many teen girls don’t mind this, it can be a little overwhelming.
This is slightly more obscure slang, rather than slang you hear every day. “OMG”, on the other hand, is one you hear everyday. “OML” could also mean “on my life”, so it’s best taken in context, especially when the person you’re speaking to usually gives you a string of abbreviations in text or online chatting.