We studied the performance of an OCT imaging modality on the task of detecting an abnormality in biological tissue. Optical propagation in biological samples is dominated by scattering due to fluctuations in refractive index. We used the first order multiple scattering approximation to describe the scattered field from the tissue. The biological tissue was described by it permittivity field and the corresponding scattering potential. The normal state of the tissue (the background) was modeled as a spatial Poisson field of randomly distributed scattering centers, and the abnormality (the target) as a region with a higher concentration of scattering centers embedded in the background. The target detectability was then calculated using a quadratic observer. We considered the effect of fluctuations from the broadband source, the shot noise fluctuation of the imaging system, and the scattering noise due to refractive index fluctuation in the biological tissue. We also studied the detectability of an embedded abnormality in biological tissue with respect to to size of th abnormality.