Once the piston is protruding from the body, you are ready to replace this final o-ring. You can choose to leave the piston in the body, or remove it, the shaft, and the spring. The choice is yours.

Pictured here is the pump completely disassembled, with the piston removed, for reference.

Remove the o-ring on the piston like you did the others. Find a suitable replacement.

Lubricate both the o-ring and the inside of the piston cylinder with a light coating of synthetic grease.