No, you can not use O-Rings. They will simply crush and squeeze out without sealing.
You do not need to use the stock blue seals. These seals have been standardised for many years and on many different engines. If you can not find them at your Mercury dealer try OMC, Honda, or Yamaha. Color does not matter. And Metric is so close to SAE that the foreign ones will fit. You can even use the old Chrysler white nylon crush seals.
As a bit of added schooling, years ago OMC used a screw with a groove under the head to accept an O-Ring. This prevented it from overtightening the ring while still sealing. They may still be available.