The OMA alleviates your responsibility, if there is any doubt, contact Medlink.
I'm not a doctor, nor is any of the cabin crew. I'm not completely heartless though, one time a few years ago we were parked out on C stands doing a night india turn to god-knows-where when an elderly lady became short of breath and pale. We called medlink and as you guessed... ...they recommended offloading her. Initially the CSV was concerned, which is why we called medlink, then the woman protested that she was fine when given the bad news. It was beginning to turn into a situation and I went back, she looked fine to me and the purser agreed. As it turns out, she got out of breath coming up the loading stairs, which can be a task for an elderly person, let alone any one of us out of shape captains!
I actually called medlink back and told them she looked ok to travel. They had us take her blood pressure, which was normal, then said she's fine to travel.
PS. I also didn't want to wait 45 minutes for them to find her bags
CC