Not all O-rings need greasing. Some brands may actually deform because of greasing and destroy your camera.
For example, in Ikelite housings the main O-ring (that seals the back cover) and the Strobe's battery lid do not need greasing.
And even those who do need greasing, wrong type of lube may destroy them too. Better follow manufacturer instructions.
At home I remove the O-rings (cover and port), put them in separate zip bags (because the port O-ring is slightly lubricated), put them in the houising and close it (cover and port).
I leave the strobe battery compartment open.
During "diving sessions" I do not remove the camera from the housing nor any O-rings. I only open the cover or port to change film/lens and close again. I prefer to do it in a clean place (hotel room, etc.) and not on the shore. I keep the camera like this for several days. IMHO it is better to keep them a few days in their place but clean rather than removing and inserting them again, increasing the chances to get the !@$#$*%@ piece of dirt that will leak.