Art of Problem Solving (AoPS) is one of the most well-known websites internationally for preparation in mathematics competitions.
The website is available here: 🔗 https://artofproblemsolving.com
The portal is in English and contains a large amount of material dedicated to olympiad mathematics and problem solving.
Not all content is free, and part of the website is devoted to paid courses.
In addition, the platform also covers many topics that go beyond the scope of the Italian Mathematics Olympiad.
Despite this, AoPS remains one of the richest online resources available for anyone who wants to deepen their study of competition mathematics.
Video lessons
A particularly interesting section is the video lessons, available here:
🎬 https://artofproblemsolving.com/videos
This page contains many educational videos explaining problem-solving techniques, strategies, and typical topics from mathematics competitions.
Collections of international competitions
AoPS also contains a large collection of problems from mathematics competitions in many different countries.
In particular, in the section:
https://artofproblemsolving.com/community/c13_contests
it is possible to find the problems from olympiad competitions from many nations, organized by competition and year.