[kaiser]

Of course the O-rings seal. That is what they are there for.
They force the liquid/gas to go through the cap, so that the valve inside the cap will work.
The flat rubber on the top seals against the outside, so a seal both ends, and the parts inside the cap can do their stuff.

As for the O-rings being damaged by one solid close, nonsense.
Damage will take place, but that is a long term process owing to abrasion and plastic deformation.