For those of you who don't know, which is most likely all of you 'cuz I'm probably the first person to ever do this, O.E. stands for Octo Expansion, a new single-player campaign that was recently added into Splatoon 2. I'm not gonna spoil anything, but upon beating the Octo Expansion, you get, upon other things, the option to play as an Octoling instead of an Inkling