The poorly puffin passed away despite being cared for by the GSPCA over the space of two days.
He was rescued more than a week ago and was the first of his kind to be found in Guernsey since 2014.
However, experts say he had ingested too much oil after arriving coated in the substance and it is 'almost impossible' to rehabilitate birds in that condition because they try to clean their feathers.
He died two days after his rescue.
GSPCA manager Steve Byrne says it's an incredibly sad moment.
"We were all incredibly upset when we were unable to save Oscar Puff the puffin.
It is rare for puffins to come into any rescue and when any bird comes in oiled they are often poisoned from cleaning the oil off themselves.
We strive to help over 2000 wild animals and birds every year."
Annual Guernsey Scarecrow Festival set to raise thousands for charity
Channel Islands face 'physical' Poland test
Baby born on Guernsey's Marine Ambulance
New interim leader for Guernsey's healthcare system
Guernsey's reservoir levels falling after prolonged dry spell
Guernsey's Commonwealth flag carrier in early Games win
Abundant year for jellyfish in Guernsey waters
Battery fire ignites in Guernsey waste processing centre
