If you have qualified for JMO/AMO, then congratulations, you are top 500 best high school mathletes. It is also a bottleneck for a lot of students as schools do not teach proof and to qualify for MOP, you have to grasp the skill of proof.
My friends in MOP built a Channel and provide free videos teaching math and these videos are very helpful. Here are some videos. If you have any questions, I am available to tutor this level of math.