A study of the association of breast cancer with duration of oral contraceptive (OC) use compared a group of women with breast cancer with a group of women of similar ages who were admitted to the same hospital for non-cancer diseases. From interviewing the subjects it could be determined who had used OC in the past and for how long. The following data were gathered.

OC Duration Breast Cancer Hospital controls
Never Used 235 273
< 1 year 27 26
1-4 year 43 29
> 4 years 46 23

Calculate the appropriate measure of association for this case control study and the 95% CI for each level of OC duration, using never users as the common reference group.