In a glass storage container add the olive oil, vinegar, oregano, salt and pepper. No need to mix the ingredients together.
Slice onion thinly, add onion rings to the marinade.
Close the glass container with the lid, and shake the onions in the sauce.
Leave the onions on the counter for 1-2 days, flipping the container over every 6 or so hours to give every onion piece a chance to absorb those flavors.
As the onion sits, the juices will release giving you more liquid.
After 2 days, refrigerate the onions for up to 5 days.