I'd never before heard it claimed that "Op Ed" was short for "Opposite Editorial", and had presumed it was short for the catchall "Opinion / Editorial". But you are correct: https://english.stackexchange.com/questions/204681/op-ed-or-....

It seems worth clarifying, though, that "Opposite" is referring to the layout of the page (ie, the "facing the editorial page"), and not to the position in the debate (not "arguing against the editorial position"). The two earliest first cited usages in the OED makes this clear:

  1924   R. H. Lyman World Almanac 25/1 
    This ‘op. ed. page’ (page opposite the editorial page) is
    unique in American newspapers. It is a page of opinion in
    all the arts.

  1931   J. W. Barrett World, Flesh, & Messrs. Pulitzer iv. 82      
    Swope developed..the idea of a distinctive ‘opposite
    editorial page’ consisting of two ‘columnists’, one book
    reviewer and the daily output of the dramatic critic. We
    called this the ‘op-ed’ for short.