I found oil under my CAS (Cam Angle Sensor). What should I do?

Step 1: Replace the o-ring.

Mitsubishi Parts:  
MD619648 would be used on a 90-94 Style sensor. 
MD622021 is just for the 97-99 DSM/EVO 8/9 sensor.

Aftermarket:
Felpro #: 420

DIY: 
CAS o-ring size is P30 (w=3.5mm, ID=29.7mm)

Autozone also sells a small pack of "Distributor O-rings". There is one in there that fits perfectly and has worked for me. It come sin a pack of 8 I believe, varying in sizes.

Go to Lowes / Home Depot and get some pluming o rings

If still leaking after replacing o-ring, keep reading.

Add RTV between the cam position sensor support and the head, and new gasket (MD329503) and or RTV between the support and cover

JNZTuning writes: http://www.dsmtalk.com/forums/showthread.php?t=235353#9

The gasket on the front of the housing (3 bolt) is part number MD329503. If it's not too severely damaged/worn, you could always try a *THIN* coat of RTV to patch it in the meanwhile.

You already have the other seal for the CAS itself (MD622021), so the only other seal is on the back side where it seals to the head. If you're getting leakage out of the front cover, I would probably not only fix the 3-bolt cover gasket, but reseal the RTV on the rear where it sits against the head.

I grabbed a used one that was sitting back in the parts bin (please excuse the child-like use of Paint). You can see in the 2nd picture (arrow) where the groove is that you need to RTV. 

Make sure that you clean off both surfaces (head and CAS) to remove the old RTV and oil.

You should be oil-free after that.

(download archive copy w/ images here)