According to the Parts List there are three O-rings that I feel are vital to the filter area.
There are two thick, large O-rings (#29) that go on either end of the filter cartridge.
These seal the ends of the filter to prevent the oil from bypassing the filter.
There is one thin, large O-ring (#35) that goes in the groove on the cover. This is the primary seal for the filter cap.
There is a paper flat gasket that also seals the cover but I consider it to be a secondary seal as usually these flat gaskets under pressure do a poor job of sealing.
The flat gasket could be there to lower the spring pressure to lower the point where the pressure relief valve opens and if so, that's a good reason to leave it in place.
The other smaller O-ring (#34) is shown going between the spring for the pressure relief valve plate (#31) and the oil filter cap (#37).
This appears to act as a buffer between the spring and the aluminum cap, probably intended to prevent the hardened steel spring from wearing a hole thru the aluminum cap.
If your motorcycle is equipped with the self contained filter cap and pressure relief valve assembly, you would never know this O-ring was there under normal servicing.