counting question

I hope this is a pretty easy one, but since I'm brand new to Excel, it not for me. Let say I have a colum of x's and o's. All I want to do is count the number of times an O's appears before an x, such as this:
x
o
o
o 3 o's appeared
x
o
o 2 o's appeared
x
x
o 1 o appeared
x

There might be a column of 300 hundred of these x's ans o's together, so it will output numerious times beside the column.. Would like it to output exactly like the example, in the coulumn right next to the column of X's and O's. Would like to stick to just formulas not vb, because I'm a newbie Thanks in advanced for your help