If you are olive/yellow like me, then you need foundations which are yellow based. I use MAC's Studio Fix Fluid in NC 44. It's more yellow and blends into my skin so well that I can barely tell that I'm wearing it. But I had the makeup artists try a few shades on me first, before buying on that worked.
Another brand of foundation that I like is Stila. That have some yellow in their foundations. I believe the last one I tried was Stila Illuminating Liquid Foundation in 50 W. The W stands for Watts. 50 watts is the shade best suited for medium complexions with yellow undertones. 60 and 80 watts is best suited for more darker complexions with yellow undertones. So, if you'r not dark and more olive, I'd go with the 50. You can find Stila at Sephora! Actually you can find anything (except for MAC) at Sephora! Just go there and tell an artist what type of foundation u're looking for and have them apply and match it to your skin. An expert can recommend a matching foundation suited for oily skin.
I do always ask the people working in Mac to help me out but one lady gave me a match which was exactly my color but just after looking at other things in the shop when i looked at myself in the mirror i looked really dark..like muddy..and then i asked another one to help me out and she was like since you have a oily skin use a foundation thats one shade lighter then your shade that way it wont make you look dark i did that and now all of a sudden i am not liking it :s it makes me look weird in pictures..i don't know if the camera is bad or the foundation..but yes i still get dark every time apply anymake up and when i wash my face people are like fair and lovely say moo dho kar aayi ho ;S which is pretty embarressing you know..