Oxford is a more rugged fabric woven from a heavier yarn than used in poplin. The threads in the warp (the vertical threads in the weave) are yarn dyed and are not as thick as those in the weft (horizontal threads), which are left a natural shade of white. The result is a heavier yet soft fabric. Oxfords are more durable weave and slightly heavier the Twills and Poplin fabrics.
These fabrics are created using rougher, more substantial yarns, and hence are inherently more casual. Oxford fabrics are created using a basket weave, where multiple yarns pass over one and other in vertical / horizontal directions. As a more casual fabric, its most natural form is the button down collar. In colored and patterned Oxford shirts, only the threads running in one direction are dyed, while the others are left white. This gives the fabric its characterized textured appearance. As a result of these coarser yarns, Oxford fabrics are generally more affordable. An Oxford shirt is probably not the right choice for evening wear or a more formal office, but it can make for a great casual / weekend shirt.