The correct city name is Porto, I have no idea why they call it Oporto.
In Portuguese it was usually called Cidade do Porto (city of the port), which was shortened to be just Porto.
In Portuguese it was usually called Cidade do Porto (city of the port), which was shortened to be just Porto.
Nothing modern uses this name anymore, though. I always see "Porto", even in American English.
But, it's a recent enough fix that you can still find signs of it. For example, you can book a flight to "Porto (OPO)". Just like you can book a flight to "Beijing (PEK)".