The o-ring should should have been on the inlet joint. It's the joint facing the front of the car with the big hose that runs to the front of the car near the reservoir itself (and in front of the belt).
If you did change the correct one out, are you sure it's not just residual air bleeding out? The fluid level will continue to drop as the air bleeds out. Have you done a good lock-to-lock bleeding yourself? Even though my experience is that even with lock-to-lock you'll still have a day or two where the air bleeds out and the reservoir overflows.
Did you use the revised part number and an OEM o-ring? Because if you just went with any aftermarket o-ring you're probably not going to fix the problem, especially if they're going off of the measurements of the superceded part.
If it's not just residual air, you'll need to find out where the fluid loss is occurring. There was also a recall for a power steering hose itself on these, but I assume it was already done otherwise you'd be having a fire under the hood.
