According to the OED, the newest edition of the dictionary revises more than 1,900 entries and includes a ton of new words — including the neologisms above.
So what do OMG and LOL mean to the OED? In the electronic realm, they’re merely shorthand for surprise and laughter. In the real-world space — according to the OED’s blog post — “The intention is usually to signal an informal, gossipy mode of expression, and perhaps parody the level of unreflective enthusiasm or overstatement that can sometimes appear in online discourse, while at the same time marking oneself as an ‘insider’ au fait with the forms of expression associated with the latest technology.”
The OED also reveals that these neologisms aren’t as neo as we might think: The first quotation the dictionary uses for the definition of OMG is from a letter dating back to 1917, and LOL meant “little old lady” back in 1960.
The heart sign (<3), however, is perhaps the most interesting addition. As the post says, “This update may be the first English usage to develop via the medium of T-shirts and bumper-stickers.”
One thing to keep in mind is that popular culture is major driver of new vocabulary. Once a word makes it in, it is never removed even if it becomes antiquated; the purpose of the OED is to exist as a comprehensive, living history of the language.