We address the problem of object co-segmentation in images. Object co-segmentation aims to segment common objects in images and has promising applications in AI agents. We solve it by proposing a co-occurrence map, which measures how likely an image region belongs to an object and also appears in other images. The co-occurrence map of an image is calculated by combining two parts: objectness scores of image regions and similarity evidences from object proposals across images. We introduce a deep-dense conditional random field framework to infer co-occurrence maps. Both similarity metric and objectness measure are learned end-to-end in a single deep network. We evaluate our method on two benchmarks and achieve competitive performance.