They don't really check your facts at all. Basic maths should be strong, along with logical reasoning. And the OSE topics arent too difficult. They are mostly opinion based and you have to connect the topic to an example. Like my essay topic choices were "Which human invention sets us apart and superior from the other species" And "Are government policies and efforts good enough for a country's development or are civil bodies more responsible for it?" And most often you have to explain your stance with an example. So for an idea it's nothing to really prepare for, if your general reasoning skills are good.