As a Yank living in the UK in the 1980s - OP was required wear to retain your American-ness. If you were spotted in a St. Michael’s cardigan or a C&A outfit you were… one of them!
I can assure you every American kid living in England in the 1980s had an assortment of OP shirts and Vans. The poorer among us had a few Hobie shirts mixed in as well.
Moving to America in 1986, I found that nobody in Texas had even heard of OP stuff. Time to switch to Nike t-shirts!