I have Oral B and Philips sonic and some disposable battery ones [which I have for travelling but end up giving most of them away to those staying with me - all brand new in their packets I would add]
Over the years I have bought compatible type brushes for Oral B from Boots and have had no problems with them. I've not seen any for Phillips, other than Phillips.
I think replacements heads for kids [toothbrush heads only alas!] will fit the Oral B handles, so that might be something to think about if the sensitive ones are difficult to get. Also I notice that there are a lot of gum care brushes and I imagine these are very gentle. I would have liked to try either of those but I was well stocked up on Oral B and it will be after next Christmas before I get around to it. Another possible tip - don't know whether it works - is to dip the brush in very hot water for about 10 seconds immediately before using it.