Try a website called sailboatdata.com and search for the O'Day 20. The yave pictures, drawings, and specs of all kinds for a lot of boats. Also, just search the web -- Google or Bing, for O'Day 20. You will find lots of pictures, maybe some owners manuals, reviews of the boat, and boats for sale that will also have pictures of interior and exterior.
I still do the same with my O'Day 22 and am surprised to see all the different modifications, upgrades, (and some that I would call downgrades) that different owners have tried, along with some articles on repairs to various structural points which is a great indicator of what to inspect on your own boat just because it failed on another boat just like yours.
Have fun!