Cambridge IGCSE and Cambridge O Level are separate qualifications, and the same school subject can have two different syllabuses with two different four-digit codes. Check yours before buying revision material.
Mathematics is the clearest example. IGCSE Mathematics is syllabus 0580. O Level Mathematics is syllabus 4024. They are different syllabuses with different papers, so questions that suit one may not suit the other. Revising from the wrong one wastes time.
IGCSE
O Level
AS and A Level
Every book has its first chapter free to read, so you can check that it matches your syllabus before you order.