We report a wide-field compressive diffuse optical tomography (DOT) system - optical mammography co-imager (OMCI) - which aims to augment tens of thousands of existing x-ray mammography or tomosynthesis systems worldwide by adding functional assessment of breast tissue and improve cancer diagnosis. The OMCI system utilizes large field-of-view structured light illumination and single-pixel-camera based detection techniques to produce ultra-high spatial sampling density while ensuring that the inverse problem remains compact via the development of a unique target-adaptive pattern optimization technique to achieve compressive-sensing based measurements. The reconstructed images can be further enhanced by applying a compositional-prior-guided DOT reconstruction algorithm with tissue structural priors derived from a separately acquired x-ray mammography scans. In this report, we describe the design details and performance characterization of the imaging hardware as well as DOT image reconstruction pipelines. To validate this multi-modal breast DOT system, we include reconstruction results from both tissue-mimicking optical phantoms as well as clinical measurements from normal breasts obtained from a clinical study. Sample reconstructions from a breast containing a malignant tumor are also included, showing the potential of localizing and characterizing breast lesions using multi-modal measurements combining x-ray and DOT.