On 4chan /b/, "wordfilters" used to be common. What would happen is a /b/ mod/admin *cough WTSnacks cough* would get tired of people using a certain word, let's say FURRY, and would filter the word FURRY into the word DRAMA. Thus, wordfilter. One such word that got overused is the word "repost." A repost was when someone posted a picture that had been posted before. Someone would post a picture and another would respond simply with the message "repost." A few different phrases spawned from this, the most infmaous being "everyday is repost day."
One day, the word RESPOST was filtered to the world OWL. So the phrase would become "everyday is owl day." This excited people for some reason, and owl threads ensued. People would just post pictures of alot of owls. Another common trend of the time was to shop the phrase "O RLY?" onto pictures in hopes that it would become an image macro. Well, soon enough, someone found a funny picture of an owl and shopped "O RLY?" onto it. This is the O RLY owl picture that you are all familiar with today. A few 4channers must have started posting it on other websites, and by sheer luck, it caught on like wildfire.
Honestly, it really fucking baffles me. 4chan has a history of creating next to fucking nothing when it comes to original net jokes and such. But out of that dismal place of overused catchphrases, reused jokes, and annoying camwhores came an image macro that took the internet by storm.
O RLY owl
He looked like he was saying O rly.