Fabulous quilt! I have been quilting for over 20 years and have yet to attempt the OBW. You did a spectacular job. My favorite borders for OBW quilts like yours is the original fabric. I always think it is fun to see the original fabric that made up the hexies on these. And there is no doubt that it will match!. If you have more of it or can get more of it, I would use that.
Regarding quilting, there is a lot going on in the quilt already and anything fancy won't show. As others have said, SID isn't always easy to maintain. I love quilting 1/4" away from the ditch or close to it, using the edge of my presser foot as a guide. If your batting allows it, you can just quilt around, or just inside each hexi. Another easy quilting design that would work well with this quilt is a simple cross hatch set on point. Just straight line quilting diagonally across the top, first in one direction, then the opposite, forming a grid of diamonds. I would recommend a 2" grid. Use a thread that will blend in. You can use blue painters tape as a guide for your lines. It is easy to do and quick. Best to get a walking foot for your machine for any quilting but if you don't have one you can probably get by with your regular foot, just make sure the sandwich is well basted so you don't get shifting. Many here like using diluted elmers school glue or spray adhesive. The cross hatch will make the quilt extra durable for the numerous washings a baby quilt needs to endure.