"O"-Jung.Ban.Hap. (stylized as "O"-正.反.合.), sometimes referred to as "O"-Union or just O, is the third Korean studio album (fourth overall) by South Korean pop group TVXQ, released on September 29, 2006 by S.M. Entertainment. In a year when Korean music sales had slumped, it was the highest selling album in South Korea in 2006 and won the group several prominent awards in South Korea. The album stayed charted within the top 50 throughout 2007. By 2014, it sold over 475,000 copies.

The album's name "O"-Jung.Ban.Hap, literally translated to "O"-Justice.Opposition.Solution, is based on Georg Wilhelm Friedrich Hegel's theory of thesis, antithesis, synthesis. The album's mononymous lead single has lyrics that regard global conflicts with a plea for peace and resistance. The album has a balance of fast tempo songs and ballads.

The album was released in 4 different versions and packaging, versions A and B, C and D. Versions A and B promote the title song """O"-正.反.合.", while versions C and D promote the fantasy theme of the song "Balloons" and contain new songs and a DVD in each. The initial versions of the album with the original track listing was released in CD and cassette formats.

For the version D, the label company did a unique box packaging, with special deck of cards with members' pictures wearing costumes included with the CD and DVD specials.

In Hong Kong and Taiwan, the album was released under the names "O"-Jing.Fan.Hap. and "O"-Zheng.Fan.He. respectively, while in Japan the name of the album is "O"-Sei.Han.Go.. The Japanese release does not contain the repackaged versions that are available in the Korean release.

The lead single is a fast-tempo fusion of rock, hip hop, and electronic trance. The music video took several days to complete as its footage was shot in several different countries. U-Know Yunho's footage was shot in Japan, Micky Yoochun's and Max Changmin's in Prague, Hero Jaejoong's in Thailand, and Xiah Junsu's in South Korea. The music video showcases intense and unique dance choreography with stylized outfits.

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