I am a recent owner of a 222. I don't know much but pre-1984 they were model 22s. In 84-89 they built the 222 model with the overall difference in being a fractional jig sail. I guess easier to sail. Also the interior layout in the 22 was more compartmental where as the 222 was all open.
You might find more info in the boat info > owner modifications > look at the 22 info and then the 222.
Most people agree that the 222 is better than the 22, I guess for it being more modern and updated. In the end, find a good example boat and go sailing.
Here is a long thread discussing it -
http://archives.sailboatowners.com/pviewarch.htm?fno=9&sku=2008078072103.6&browse=T&g=[g]&ptl=O%27Day%2022%20vs%20O%27Day%20222%20-%20which%20is%20better%3F