In a pot, combine salt, sugar, vinegar and oil. Bring to just a boil, stirring to dissolve sugar and salt. Add onions and return to a boil.
Using a slotted spoon, pack onions into canning jars. Pour in pickling liquid to cover. Seal jars with lids and refrigerate for at least 3 days to blend flavors. Store in the refrigerator for up to 1 month.