RE: Vent O-ring
If you are sure your board is dry inside (see the current thread about "water in the hole for lot's more info" then yes, you can use an O'ring that fits form your local hardware store.
Or, you can purchase one from your Starboard retailer.
A better way to do this (since there is no "O'ring groove" in the current design of the O'ring seat and the vent plug) would be to find a fiber or plastic washer shaped "gasket" that will fill any gaps between the flat surface on the underside of the vent plug head and the flat surface machined into the vent plug fitting in your board.
Ideally you could find a fiber or plastic washer with an outside diameter that just slips into the counterbore of the vent plug fitting in your board, and with an inside diameter that's slightly smaller than the outside diameter of the tread.
I've been looking for such a washer, but as yet have not found one.
If your board is dry inside (for sure) then if you sail in one locality most of the time (the weather stays around the same temperature) then you can put in a new O'ring, or the fiber or plastic sealing washer mentioned above and leave it tight all the time.
Where you might have a problem is when transporting your board on an aircraft (not all cargo holds are pressurized and a large reduction in outside atmospheric pressure can cause the internal pressure in your board to go too high and cause some internal delamination) or when you travel over a high mountain range where the atmospheric pressure drops relaive to the internal pressure your board has been living in.
Also, large temperature changes can cause rapid heating of cooling and this will make a board with an improperly sealed vent fitting "breathe" as the temp changes.
If the temp goes up, the board will "beathe out" any pressure the interior gains, past the poorly sealed vent fitting.
Conversely, if you board is quite warm (or hot from direct sunlight or storage in a hot vehicle interior) then when you set it in the water, and the temp cools rapidly, the board will "suck" air and water into the core through the poorly sealed vent fitting.
If your board is slightly wet inside, from the damaged O'ring, you can use this "thermal expansion" and a paper towel to wick most of the moisture out of your board. Set the board in the sun, let it heat up, with the vent plug removed. The increase in internal temperature will vaporize the water inside the core of your board and push it out the vent fitting where a paper towel can be used to collect any liquid or water in vapor form and trap it outside your board.
Sealing the vent plug with marine silicone might work, but silicones, around fiberglass or epoxies is not a very good idea in my experience as the silicone "soaks" into the micro pores and akes everything very slippery and almost impossible to repair.
A non silicone product (gasket sealer from your local autoparts store) would probalby be better.
Hope this helps,