When you remove oil seal bracket (big nut where driveshaft goes into gearbox) O-ring is located on outher perimeter of this nut.
Part no is 7700532957 and it is awailable at webshops for Reanult 4. But it is plain 105x3 rubber o-ring that can be found elsewhere. I got mine at local agriculture shop among parts for tractors.
But in any case you first need to remove that big nut to change Oring and oil seal.
When taking it apart mark possition and count turns so you know how to put it back.
Turns of "nut" actually tightening bearings on gearbox final drive. They must not be lose but not to tight also.
Here are some pictures
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