From experience, since you have to drain the oil, remove the oil pan and reassemble, replacing the o-ring might solve your problem, but if not, you are doing the whole process over again to replace the sensor. I replaced the o-ring on mine (suggested by a Benz mechanic) only to find out that the oil wasn't leaking around the o-ring. The oil was actually coming out of the top of the sensor! It's not a big job but it is one you don't want to do twice and the sensor wasn't crazy expensive.
Good luck