The premise of learning the breathing, relaxation and visualisation techniques is that you practice them before the birth so that it becomes automatic - you don't have to 'remember' anything, you just go into it with a couple of simple cues that your birth partner can give you. If you're already doing it and not having a distressed birth, then it's no issue. I have a hard time relaxing normally, so I didn't want to take any chances and did my HB course, to make it sink into my brain!
Also, I had afterbirth pains for a few days, where I was taking voltaren before each BF from the uterus contracting. I had a managed 3rd stage because there was excessive bleeing and my excellent midwife (who up till then had done NO work ) decided she'd seen enough blood and it was time to intervene. DS was skin on skin the whole time. I'd had a great labour, so I don't think it's related to oxygen deprivation. JMO!