Hi I'm in year 13 doing both. Dont worry, the gap from GCSE is big, it happens to everyone. I would say use different textbooks, especially biology, Ive found that my textbook didnt have everything, to make sure you cover all the spec points use more than 1 (at least for OCR)
And just keep practising, it will come with time, especially if you're starting now and not in a few months thats really good. When you do, make sure you use the mark schemes well, youll find questions get repeated just in different forms but theyll usually be looking for the same things.
Good luck!