We have both the data for One Piece Magazine TOC and volume sales(though not all volumes) since the beginning of the series. The series was mega popular since the beginning in both forms. Getting the biggest reported 1st print of any 1st volume in the magazine, 300k. Assassination Classroom also got that same print number. And is the series with the best starting rankings any series ever had, becoming one of the "Big 3" of the Magazine series Covers in the 1st one.
Damn wow, OP was huge right at the start. I assume a lot of Jump's heavy hitters ended, right? Dragon Ball, Slam Dunk, Yu yu Hakusho, etc. all in the 2-3 years before OP started. Not too sure what One Piece Magazine TOC is tho.