I can't make out anything in the pic, but yeah i-o is Owens. As for the circle, that was where they would have put the embossing if the dairy had ordered embossed milk bottles. Unfortunately for you they cheaped out and ordered generic ones, so the glass factory put a blank slug plate where the embossing would have gone.
Could be 1933 or 43. Notice in the other one it says 47 but the 7 is lighter, it appears. The way Owens Illinois worked is the single digits meant 30s so a 4 meant 1934. Once 1940 came they either added a dot or left the single digit or added a second digit to the mold. Numbers on left are usually plant number I believe. SHA.org has great dating info on Illinois glass, Owens glass and the big merger Owens Illinois. I have had the good fortune of digging lots of intact embossed milks in my 1900s local dump.