LO (Laughing out) is a consequence of Bloodlust80 forgetting to add the extra L in LOL. However since then, it has taken off as a legitimate phrase on the Internetz
Example of usage
person 1: I once said something crazy
person 2: LO
Person 1: What are you doing right now?
Person 2: Browsing reddit
Person 1: LO
"This is a funny article LO"