The real question is what is your passion. what will make your eventual career secure is that you can research things that you really care about. doing a phd is tough enough, and if you do it just for some virtual job security - then you are in the wrong profession.
typically, beside the regular background you only need keen interest in a field, you don't necessarily need to have studied it in the past. to show that you should read some papers in the field, read about the field to get a bit of an overview (wikipedia does a great job), then come up with an interesting question that you want to study and write a paragraph or two about it in your SOP. why it is interesting in general, why you are interested in this topic, how you'd go about study this topic, and how it relates to the department that you apply to. If you do this well, it is more than enough to show interest.
I know these fields seem very far apart but I really am struggling with choosing between the two. I find management/OB to be more interesting on the surface than Finance. Also, I'm nervous about going into a Finance Phd and competing with a bunch of geniuses. But I feel like Finance would be a more secure career as far as getting a job after graduation, and having other options besides just being a professor.
My questions are... what are the different requirements for applying to management verses Finance. any specific math courses, stats courses, etc. Also, I do have a real interest in org. behavior but I don't work in the field and did not major in psych or management so how do I prove to the admissions committee that I have an interest in the field? I need advice!!!!!!