Pop star Justin Bieber revealed his friendship with Psy online.

Yesterday, Justin Bieber tweeted, "Heyyy Sexy Lady," which was a part of Psy's "Gangnam Style."

Psy then replied to his tweet with, "op op op," showing their friendship. Justin Bieber had tweeted this message to show his friendship with Psy.

The two met through their manager Scooter Braun who found the "Gangnam Style" music video on YouTube.

On the other hand, Psy is currently in Australia finishing up his activities there. He will be back in L.A. on October 19.

Psy has recently become a global sensation with his song, "Gangnam Style." He has ranked number 1 on iTunes in 30 countries and has ranked number 2 on the U.S. Billboard main single chart HOT 100.

He has also surpassed 490 million views on YouTube with his music video for "Gangnam Style."

He has appeared on many shows such as the "Ellen Show" and "SNL" and featured on news programs such as NBC and CNN.

He has also recently signed a contract with Justin Bieber's agency and plans to release his first U.S. single sometime in November.