A very good friend of mine regularly sails his centerboard-model O'Day Mariner on Lake Michigan, which I would not equate with open ocean sailing, but I would say has the potential to equal or exceed the conditions one might find sailing in large open ocean bays. I've joined him on many occasions and have found the boat a delight to sail in waves and wind. Admittedly, we haven't been out in 8 to 10 footers but I'm not sure you want to be out in Lake Michigan in many small boats in those conditions.
On the big lake, we generally sail with the hatch boards in. While I agree with an earlier poster that the cockpit drains are under-sized, I note that the cockpit is relatively shallow. In terms of reefing, my friend intends to add that capability this year as his boat didn't come with it. The boat could probably stand for a reef at 20 kts of windspeed or so.
For whatever it is worth, by this eye, I find the Mariner to be among the prettiest of the small trailer-sailers. This may have something to do with my understanding (which may be incorrect) that the hull design was based upon the Rhodes 19.