Silicone lubricant grease. Protective, waterproof, simple to use.
As o rings are made from synthetic rubbers such as Neoprene, nitrile, and EPDM, you can use petroleum grease to perfectly lubricate the o ring.
However, knowing if the o ring is made from ordinary natural rubber or synthetic rubber is where the problem lies, as petroleum will degrade natural rubber.
To avoid such problems, we’d recommend the use of silicone lubricant for seals and o rings.
Waterproof and lubricating, silicone lubricants err on the side of caution.