Here is a link to revision resources used by/suggested by my students:
Quizlet – flashcards on Topic 1-7 – Thanks P. Sudhakar!
Notes on Topic D – OOP (look at the last column!) – Thanks D. Mulkey!
Notes on Topic 4 – Algorithms (look at the last column!) – Thanks D. Mulkey!
Option D (OOP) Revision Guide – Thanks C. Ghali!
Topic Test – testing all major topics in IB CS
Notes on topic 5, Paper 2 Option C and Paper 3 – Thanks M. Guse!
Summary of command terms (important to know for deciphering questions)
Recursion
Recursion is an important but tricky part of the curriculum. It features in both Paper 1 (in Topic 4) and in Paper 2‘s OOP option (in Topic D4).
Here are a few links to some resources that explain this concept in more detail:
Wikibooks A-Level explanation (also applicable to IB CS HL)