Oppo phones don't go on sale in the US or Canada, but that's not because of any ban on them. Instead, Oppo simply doesn't choose to sell smartphones in the US - it's a challenging market, and one currently dominated by Apple and Samsung, which makes it hard for even huge foreign companies to break in. Oppo's sister company OnePlus does sell phones in North America though, so you should look to try a OnePlus phone if you really can't find anything from Oppo.