Yes the seal can be field serviced, no it is not an oring, (there may be an oring as an integral part of the lip seal, typically raw water
pumps have 2 roller bearings that are not affected by a water leak.
You can find a thread about this by Belizesailor by searching this topic here on CF:
Quick Raw Water Pump Sea Water Seal Replacement
I would only do this as a quick fix, may be just me, but I like to clean everything up and check the bearings, polish the shaft and change the oil seal
and impeller all at one go.
Also be aware that if you change the spring in the seal with an o ring, the o ring typically has a shorter life than a spring and will cause the seal to leak as it hardens, the seal lip may still be fine...
One should keep a couple seals
and an impeller for spares for just this occasion (and learn the way to change them). It's a good idea to change them every 3 years or so, to keep your spares fresh (they deteriorate with age).