International Chemistry Olympiad (IChO)
An International Information Centre of the International Chemistry Olympiads is located in Bratislava, Slovakia.
It collates all the information and documentation of the IChOs from the beginning of the Olympiad to the present.
Official Webpage
Past IChO tasks
https://www.icho.sk/competition-problems/
Past Preparatory problems
https://www.icho.sk/preparatory-problems/
National Papers from other countries
U.S. National Chemistry Olympiad