Start Here:

• Geoff Smith's Advice for young mathematicians — https://people.bath.ac.uk/masgcs/advice.html

Top Books:

• Problem Solving Tactics — https://shop.amt.edu.au/products/pst

• Art and Craft of Problem Solving — https://www.amazon.co.uk/Art-Craft-Problem-Solving-3E/dp/1119424992/ref=pd_lpo_1?pd_rd_i=1119424992&psc=1

• Euclidean Geometry in Mathematical Olympiads — http://web.evanchen.cc/geombook.html

Books and Handouts:

• Olympiad stuff you should know   (Markus#7658 and Joe 1#3988, 2019)
• Problem Solving Tactics   (Angelo Di Pasquale, Norman Do, and Daniel Mathews, 2014)
• Mathematical Olympiad Dark Arts   (Adam P. Goucher, 2012)
• The Art and Craft of Problem Solving   (Paul Zeitz, 2007)
• Putnam and beyond   (Razvan Gelca and Titu Andreescu, 2007)
• Problem solving strategies   (Arthur Engel, 1997)

Combinatorics:

• Olympiad Combinatorics Book   (Pascal96 (AoPS), 2018)
• Concrete Mathematics: A Foundation for Computer Science   (R. Graham, D. Knuth, O. Patashnik; 1994)

Geometry:

• Euclidean Geometry in Mathematical Olympiads   (Evan Chen, 2016)
• A Beautiful Journey Through Olympiad Geometry   (Stefan Lozanovski, 2016. Updated 2020)
• The Method of Moving Points   (Vladyslav Zveryk, 2019)
• 100 Geometry Problems: Bridging the Gap from AIME to USAMO   (David Altizio, 2014)

Number Theory:

Modern Olympiad Number Theory

 (Aditya Khurmi, 2020)

Other Handouts:

• Handouts from national camps and elsewhere — https://www.dropbox.com/sh/w9mfy9qtjs68xzc/AADnnQKWONBsboMGVDiuS-kAa?dl=0

• Useful Theorems and Proof Techniques — https://drive.google.com/file/d

• Cut the Knot — http://www.cut-the-knot.org/

• Power Overwhelming — https://blog.evanchen.cc/

• Olympiad geometry problems with AOPS links — https://imogeometry.blogspot.com/p/geometry-solved-problem-collections.html

Youtube Channels:

• 3Blue1Brown — https://www.youtube.com/channel/UCYO_jab_esuFRV4b17AJtAw

• Michael Penn — https://www.youtube.com/c/MichaelPennMath

• RedPig — https://www.youtube.com/c/RedPig_maths_olympiads

• Shefs of Problem solving — https://www.youtube.com/c/ShefsofProblemSolving

• The AoPs youtube channel — https://www.youtube.com/user/ArtofProblemSolving

• vEnhance — https://www.youtube.com/c/vEnhance

• TheBeautyofMath — https://www.youtube.com/c/TheBeautyofMath

• Dedekind Cuts — https://www.youtube.com/channel/UCVrSVkLq5HHVYbwgA9Nt8HQ

• MODS — https://www.youtube.com/channel/UC025OYUBAHm5vWAnzvIkdug

• MindYourDecisions — https://www.youtube.com/c/MindYourDecisions

• Way Tan — https://www.youtube.com/c/waytan/featured

• letsthinkcritically — https://www.youtube.com/c/letsthinkcritically

• Michael Greenberg — https://www.youtube.com/channel/UC3mhbGC7kQgzkXT9fceNOwA

• John Liu — https://www.youtube.com/channel/UCimJMDlPOc6vDoZlAJKSyXg

• mathmemo — https://www.youtube.com/channel/UCYeFBc0D8W-9dMUzoPUOn5w

• ProfOmarMath — https://www.youtube.com/c/ProfOmarMath

• Way Tan — https://www.youtube.com/@WayTan

 "a channel that solves math olympiads and has lessons/mock tests/explanations to prepare those taking math olympiad"

• Calimath — https://youtube.com/@calimath6701

Other Stuff:

• IMO website — https://www.imo-official.org/

• AoPS Contest Collections — https://artofproblemsolving.com/community/c13_contests

• IMOmath — https://imomath.com/index.cgi

• AoPS FTW — https://artofproblemsolving.com/ftw

• Problems in spanish — https://proyectomuro.com/

• AMCTrivial problems trainer — https://amctrivial.com/

• Daily Challenge with Po-Shen Loh — https://daily.poshenloh.com/

• An IMO results visualiser — http://olivernash.org/

•Literally thousands of maths books — https://docs.google.com/spreadsheets

Articles:

• Time Management
• Pros and Cons of Math Competitions   by Richard Rusczyk  .
• Establishing a Positive Culture of Expectation in Math Education   by Sister Scholastica Award   winner Darryl Hill.
• Stop Making Stupid Mistakes   by Richard Rusczyk  .
• What Questions Really Are the Stupid Questions?   by Richard Rusczyk  .
• Learning Through Teaching
• How to Write a Math Solution   by Richard Rusczyk   and Mathew Crawford  .
• Inequalities   by Dr. Kiran Kedlaya
• Olympiad Inequalities   by Thomas J. Mildorf
• Olympiad Number Theory: An Abstract Perspective   by Thomas J. Mildorf
• Number Theory   by Naoki Sato
• Olympiad Number Theory Through Challenging Problems   by Justin Stevens
• Barycentric Coordinates in Olympiad Geometry   by Max Schindler and Evan Chen
• Lifting the Exponent (LTE)   by Amir Hossein Parvardi
• The uvw Method   by Mathias Bæk Tejs Knudsen
• The Chinese Remainder Theorem   by Evan Chen
• Contest Reflections   by Wanlin Li

source: 

, https://artofproblemsolving.com/community

, https://artofproblemsolving.com/community/c89
, https://web.evanchen.cc/are
 w

Please drop all relevant resources for IOQM,RMO,INMO including Books, Notes, Lectures, Tests, Question Papers.

I will start by recommending books like-

1. An Excursion In Mathematics- A crisp and thin book especially for RMO and INMO basics. It is more or less sufficient for IOQM. There are all four major portions(NT,Algebra,Combinatorics,Geometry) of mathematics in it in a good quality.

2. Challenge And Thrill Of Pre-College Mathematics- A book especially known for geometry. The geometry from this book is more than enough for IOQM,RMO and INMO basics. The geometry of this book is in 2 parts and the first part is absolutely necessary and very basic and doable and the second part contains a lot more extra syllabus for ioqm and rmo. People generally dont recommend other chapters from this book but u can try if it suits you.

3. Mathematical Circles- It is a thin and basic book. It is not designed for completing IOQM or olympiad syllabus but the questions the book has really focus on improving thinking skills. I recommend this to younger people. Although the questions are basic and wouldnt be ever asked in Indian olympiads. But the foundation it may build may take you to places.

4. Pathfinder for Pre-RMO/IOQM- For jee students i emphasise this book as it is the bridge between ioqm and rmo. The questions and the theory is of IOQM level and this book is absolutely enough for people who are good in jee maths and are looking for a quick alternative to prepare fast for ioqm.