what causes the green light on the a/c switch to flash ? on 2009 Mitsubishi Galant

the a/c then starts to blow hot air ,i also notice oil in the area of the compressor .

It means most, if not all, of your R134 refrigerant has leaked from the system - A/C light flashing means the compressor is not even kicking on because it is detecting either little to no pressure due to lack of refrigerant. I've been dealing with this on the same exact vehicle for about 2 years now. First I replaced the condenser because I found a couple forums where some people stated that a lot of them from this year are notorious for leaking near the top left (facing the grille) welded area of the condenser, plus I added some of that fluorescent a/c oil to the system to detect any future leaks. The shop said it held a vacuum for over 30min, but all of my coolant leaked out again within a month and I couldn't find any leaks using a black-light. I got a giant can of R134 and refilled it again, and this time on a whim poured a tiny bit of the dyed oil into the low-pressure valve stem and immediately saw a bubble come up every 20 seconds or so for a long time. So next time it runs out I will replace that Schrader valve and see if that fixes it.

Sorry for rambling, but I just like to give a lot of detail, so hopefully this helps - I know firsthand how frustrating a/c problems can be. Since you mentioned seeing oil around the compressor, you may have a leak where the hoses attach, but it could be something else. Good Luck!