I've never read OPM but I'm pretty sure he got a little cameo in the Invincible series : r/OnePunchMan

Not the Heroes who are Most Deserved, Just the ones who are Most

Saitama vs Goku: And the winner is… – The Observer

Onepunchman Stories - Wattpad

BadJohn's Speakeasy — Ever have that moment where you're excited

Manga Spoiler] The Number 1 is back for another badass cameo

Reading some one punch and you come across yourself.

Magpie's Nest — Interpretation is free, facts are sacred

One Punch Man – A Hero Nobody Knows (PS4) reviewed. - The Technovore

kingkira96 – Page 4 – KINGKIRA'S CASTLE