Add warm tap water to a heavy-duty pot or heat-proof container. Attach the sous vide cooker and set to 82.2ºC/180ºF if using a bag, or 60ºC/140ºF if using a litre/quart jar.
Prepare the onion:
Peel and cut the onion thinly and evenly into half moons, using a sharp knife or mandoline slicer.
Make the brine:
Combine the vinegar, water, sugar, salt in a measuring cup or bowl. Add the onions to the bag, sprinkle the oregano on top and add the brine. Use the water displacement or vacuum to seal the bag. Or add all the ingredients to a jar. See tips section above.
Pickle the onions:
Add all the ingredients to a jar or bag (see above for tips), seal and submerge in the heated water. If using the higher temperature and a bag, cook for 30 minutes. For the litre jar option, cook for 2.5 hours at the lower temperature.
When ready, cut the bag open and transfer the contents to a large jar. Allow to cool at room temperature, then seal and refrigerate. Or remove the jar from the water, allow to cool at room temperature before refrigerating.