If clear water is leaking into the coffee collection area or through the bottom of the main body of the device it probably means that any or all of the O rings may require replacement.
Although failure of the large O ring is most likely to be responsible, it is recommended that, for convenience, you buy this kit with the additional O rings which may also require replacement.
The large O ring replaces that in the spring-loaded section of the Brewing Unit which holds the coffee pod. This is the most likely to fail as it is subjected to high pressure and temperatures and movement, which limits the life of the seal. It may also deteriorate if the device is not in regular use. (see the third photo)
The single medium-sized O ring replaces that which seals the tube delivering superheated water from the heating block to the Brewing Unit (photo no.4)
The two smallest O rings fit either end of the plastic pulse dampener (see photo 5) which carries room temperature water from the high pressure reciprocating pump to the heating chamber.
There are many videos on YouTube (of variable quality and value!!!) showing how to disassemble the Nespresso devices and replace the seals.
I recommend following those uploaded by Adrian B during the last year or so.