Should a new rear shock be building oil in the shaft? I've ridden my new bike (first full suspension I own) for about 150km now and I'm seeing some oil building over the O-ring in the shaft. It's a lot more oil than what you normally see on a front suspension, so I'm starting to worry about it, but it can be just my ignorance :p

Is this normal or should I send it back to the manufacturer? it's still on warranty.

There is no dripping while stored, so it's maybe only about wiping out the excess after every ride. Or the few first rides?

The shock is a RockShox Monarch RT
That is definitely not right. Send it back for repair.  

Similar to front shocks, a good shock will have a very light film of oil you can just feel on the surface. A wipe with a clean cloth will show no visible sign of oil on the cloth (maybe smudge of dirt).  In this case, its clear an oil seal has gone. 

Side note: It looks like you could do with lower air pressure for the riding you are doing. You have no used anywhere near the full travel of the rear suspension.