To expand on this, Japanese kanji can be sorted a few different ways. The list in the index of O’Neill’s Essential Kanji first sorts them by total number of strokes (this is one reason why knowing the proper way to write each character is important; native speakers get tripped up on this a lot, especially with the prevalence of automatic conversion software). Kanji dictionaries for native speakers may omit this step.
It then sorts them by the type of primary radical (many characters are made up of multiple smaller elements, but one is recognized as the ‘defining’ radical): whether its left-half, top-half, surrounding, or “other” (such as when the entire character is one radical).
Within that division, they are then sorted by the number of strokes in the main radical: First left-hand radicals from 1 stroke to x strokes, then top-half radicals from 1 stroke to x strokes, etc. Although there is an official “sequence” for the radicals which is listed in the back of the book, O’Neill doesn’t use it to order radicals with the same number of strokes.
I don’t know how kanji are further sorted beyond this point, but by then it’s usually narrowed down far enough that picking out the one you’re looking for is pretty easy.