The postmark rule refers to a law passed in 2021 that allows ballots mailed* on Election Day to count even if they are received up to seven days after Election Day. Yes, you can mail your ballot on election day so long as it is picked up and postmarked by the post office that day.
This improvement ensures that every vote cast on time gets counted. It may also delay the results of close races because it will take a few days for all the votes to be counted.
*OAR 165-007-0045 requires ballots to “have a postal indicator” showing that the ballot was mailed on or before election day. Postal indicators, or postmarks, are applied to mail when they are processed at the post office.