Why is my car cranking and then dying?

I have a 2002 toyota sienna. I tried to crank it, and it would crank and then immediately die. I had it towed to my mechanic who said that it was now running fine. He said that it was probably the fuel pump. So, he told me to take it home. Also, before all this happened my check engine light came on and we took it in and had it put on the diagnostic machine, and it is because of the o2 sensor. My mechanic told me i could drive it with that problem. I dont know what to do. Is there any way to make sure that it is actually the fuel pump before i fix it, because it is really expensive to fix. Thanks