This megalist was only possible due to aops's lovely community

Geroff smith's advice for young mathematicians: https://people.bath.ac.uk/masgcs/advice.html

aops wiki: https://artofproblemsolving.com/wiki/index.php?title=Main_Page

almost every popular handout for maths oly: https://artofproblemsolving.com/community/c1967976h2550333p21794712

contest collection: https://artofproblemsolving.com/community/c13_contests

resources megalist by aops: https://artofproblemsolving.com/wiki/index.php/Resources_for_mathematics_competitions

problem set for maths olympiad: https://artofproblemsolving.com/wiki/index.php/Resources_for_mathematics_competitions

Olympiad articles: https://artofproblemsolving.com/community/c13188_olympiad_articles

evan chen's website: https://web.evanchen.cc/

Yufei Zhou handouts: https://yufeizhao.com/olympiad/

Canada imo camp handouts: https://sites.google.com/site/imocanada/

Kaneda mathamatical society 🥰 resources: https://cms.math.ca/competitions/problem-solving-resources/

IMO math website: https://www.imomath.com/index.cgi

Awesome maths website: https://www.awesomemath.org/

HMMT: https://www.hmmt.org/

Berkley maths circle: https://mathcircle.berkeley.edu/index.php?options=bmc%7Cbmcarchives%7CCircle%20Archives

Alexander paaji handouts: https://alexanderrem.weebly.com/math-competitions.html

Math Olympiad notes: https://www.math.cmu.edu/~lohp/olympiad.shtml

David Altizio's website: https://www.andrew.cmu.edu/user/daltizio/mathstuff.html

inmo camp notes by rohan paaji: https://drive.google.com/drive/folders/1B6w2-TrkP-Xv8XZ6qJojkqgHqZdvYkF9