Well, personally, I am against having abbreviations like 'OMG' and 'LOL' added to the Oxford dictionary.
Having said that, I see why they are being added. On a daily basis (and yes Heiki, it is much to my annoyance too), my friends will say 'lol', pronouncing it exactly as it's read. So on that front, I might agree that 'lol' should be added to the dictionary, because like it or not, it is now a very common word in everyday speech.
However, I do not think the abbreviation 'OMG' should be added. It is not a word, when people speak it, they say 'Oh my God', not 'oh-mij'. Personally I think this abbreviation would best be left to the short-cut language of the Internet.
On a slightly related note, we should also consider those who are learning English as a second language. It would not only be confusing for them I think, but I imagine they would also find the abbreviation unnecessary. I know I would, because when you think about it, the word 'lol' is literally replacing laughter o_O.