Japan Open is smaller less prestigious event than All Japan. All Japan was a major in previous decades and featuring many big names good prizemoney but in recent years, the field is much weaker with reduced prizemoney and is more a ranking event not a major. Even then Oi has not won All Japan.
Yes Oi is high in shortlist of those who have not won major yet.
If we consider majors as W9B, W8B,W10B, Matchroom major opens (US, UK etc) and exclude invitationals like World Cup of Pool, Big Foot and exclude team events like WCP. Then for the FR top 40 (excluding less active players like Liu Haitao, Mark Gray) , IMHO this is shortlist of those who have not won a major and are due to win one. (taking into account how long they have been playing and slogging in international 9B/10B events, not how likely they are to win one

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1st in line (not in order)
Naoyuki Oi
Skyler Woodward
Johann Chua
Mario He
Lee Van Corteza
Max Lechner
2nd in line (not in order)
Anton Raga
Aloysius Yapp
Wiktor Zielinski
Kun Lin Wu
Sanjin Pehlivanovic
Roland Garcia
Alex Kazakis
James Aranas