On October 17, Psy and Justin Bieber traded interesting tweets with each other, showing their quickly developing friendship.

Bieber first wrote on his Twitter, without any intended address, “heyyyy sexy laaady.” It was obviously taken out of the lyrics of Psy’s “Gangnam Style,” and Psy soon responded to the teen singer, saying, “op op op.” Shortly after, Bieber retweeted Psy’s message, which is of course also part of the “Gangnam Style” lyrics.

As reported earlier, Psy and Bieber have become close friends after Psy was approached by Bieber’s agent, Scooter Braun. Psy has also signed a contract with Island Records, the agency run by Braun himself.

Meanwhile, “Gangnam Style” has failed to reach the top spot on the Billboard Hot 100 Chart again this week, remaining at the second spot for the fourth consecutive week. He’s currently promoting in Australia, but he’s expected to fly to the U.S. on October 19.