I wore my jeans to the OCC, but brought a small bag with my nightgown, robe and slippers. I didn't wear their gown. None of the people who were there when I was wore their gowns that evening or to sleep in. We were walking the halls so much and visiting with each other I think we were much more comfortable in our own nightclothes. I also brought gas-x strips. Plus an extra pair of underwear for the trip home the next day. You won't shower or anything the next morning, so you don't need to bring toiletries. Oh, I did bring a toothbrush and brushed my teeth the next morning, but otherwise, I traveled very lightly and it worked out fine. Make sure what you bring to wear the next morning is comfy. My jeans are loose enough that they were fine for me, but stretch workout pants would probably be perfect.
They will have pain medication for you and can handle all your medical needs, and will also give you broth, popsicles, and capri sun that night and the next morning.