Suppose we have a 2 x 2 table of frequencies 

||<tablewidth="42%">c1||c2||
 r1 ||a||b||
 r1 ||c||d||





The Odds Ratio is defined as ad/bc

It turns out that ln(OR) has a variance of (1/a) + (1/b) + (1/c) + (1/d)

so it follows

(ln OR)* ln(OR) ) / variance ( ln(OR) )

is chi-square on 1 degree of freedom.

If you have a zero cell then adding one half to all the frequencies enables an estimate of the odds ratio to be made.

Reference:

Everitt BS (1996) Making Sense of Statistics in Psychology A Second Level Course. OUP:Oxford.