I believe the model number for this plow would be 414.  This basic plow was built thourghout the 1930s and 1940s and was available as a 4 or 5 bottom plow.  
Does it have RADEX shares?  If so, it would be 1940 or later. It looks in very good shape.  The rubber tires would  suggest to me it is probably a 1940 or newer.  I am surprised that the tail wheel is not also on rubber??
There seems to be a hint of green paint on some of the parts.  Can you determine if this is original paint or has it been repainted? Oliver plows were traditionally painted red with green wheels until the late 1940s when the color scheme reversed with the plow frame painted green and the wheels painted either red or green.
It appears in the photo that there is not a spring release hitch on the drawbar.  Most all of the Oliver plows from 1946 and later were equipped with a spring release hitch. Hope this helps.