I'll keep digging, but it seems to have taken off sometime early 60's and further popularised by Kay. Although with 'structured programming' and 'object program

I'll keep digging, but it seems to have taken off sometime early 60's and further popularised by Kay. Although with 'structured programming' and 'object programs' already in use at the time (different concepts I know, but preceding OOP), the first journal mention may postdate the phrase by some years during this transition.