The Oza is sponsored by the main Japanese economic newspaper, Nihon Keizai Shinbun (Nikkei). Top prize is 10.4 million yen. The losing finalist gets about 2 million yen. All pros in the Nihon Ki-in and Kansai Ki-in are eligible.
Nowadays the title holder is challenged by the winner of a final knockout of 16 players. Entry to this knockout is from a series of three preliminaries. The first is a partial knockout for 1- to 4-dans from which 8 players (6 Nihon Ki-in, 2 Kansai Ki-in) go on to the next stage, which is two separate knockouts for these players plus the 5- to 9-dans, one in the Nihon Ki-in and one in the Kansai Ki-in.
The top 16 and 4 players respectively go to the last preliminary stage, where they are joined by winners of Rounds 1 and 2 of the previous year's event. 12 players survive from this stage and are joined by the previous year's losing finalist and the three other semi-finalists for the final knockout. In earlier years the system was broadly the same but the numbers and seedings differed slightly.
The final, held in November~December, has been a best-of-five since Term 16 inclusive. Previously it was a best-of-three.
In the early years, special matches between the new Oza and Go Seigen were also held. A parallel competition used to be run for the public called Tsugi no Itte (Next Move), and this was so popular it became a standard phrase in go.
Time limits are now 5 hours each throughout, but used to be 6 hours, with 10 hours in the final. Komi was 4.5 in Terms 1 and 2, 5.5 points thereafter.
Oza means throne.