Reselect the large "O" shape and make a copy in front (Contro + C > Control + F) then grab the Ellipse Tool(L), create a 190 by 125px shape and place it as shown in the second image. Select this new shape alonw with the fresh copy and click on the Intersect button from the Pathfinder panel. Fill the resulting shape with the linear gradient shown in the fourth image and lower its opacity to 80%.