Correct me if I'm wrong...I'm pretty sure that Riddick Bowe was the first heavyweight to have held the major 4 boxing bodies' belts (WBA, WBC, IBF, WBO) at one point or another in time. Bowe beat Holyfield for the undisputed title and vacated his WBC belt, giving it to Lewis in the process, then lost his WBA and IBF titles to Holyfield in a rematch, and after getting some ground challenged Hide for the WBO belt, successfully taking it from him, defending it once before beating Holyfield in a bout where he had the WBO strap with him, but it wasn't on the line. And second should be Tyson Fury who beat Klitschko for the WBA, IBF and WBO titles in 2015, but later took a hiatus on boxing, then returned to have a trilogy with Wilder, beating him 2/3 times and taking the WBC belt with him in the process. Then came the undisputed clash, and the third to hold all the 4 belts, Oleksandr Usyk. He became the undisputed heavyweight champion after beating Tyson Fury, having already been the undisputed cruiserweight champion, and the first cruiserweight to have held the major 4 boxing bodies' belts at one point or another in time aswell! Pretty interesting though, that it wasn't a modern heavyweight that was the first to hold those belts, it was a 90s heavyweight
The WBO heavyweight title was never respected during the era. No one would rather hold the WBO belt than be rated in the contenders of the WBC, IBF and WBA. Bowe gave up the WBO belt because Holyfield refused to fight for it if I remember rightly in 1995. A belt isn't worth much if a contender suggests the 'champion' give it up. The WBO heavyweight title gained value after Lennox Lewis retired. Even up into 2004, Corrie Sanders was relinquishing the WBO to contest the WBC. What actually happened is the WBA, WBC and IBF titles all lost value and come down to the level of the WBO. Around 2005.