Basic stats are different from AD&D ones. Only going up to +3 bonuses, no percentile strength, bonuses at lower stats for Basic while AD&D has a reverse bell curve for stat bonuses, particularly with percentile strength. OSR games have to basically pick one style or the other of those two.
Basic races do not have stat mods, classes have smaller HD and there is no stat prereqs for classes, while AD&D races have stat mods (generally only +1 -1 though) and multiple class and multiclass options while the classes themselves have slightly higher HD and all sorts of stat requirements.
Mostly AD&D has more options than basic, so usually it is adding AD&D options onto the simpler Basic base rules such as paladins, monks, and assassins and half-orcs and splitting race from class (although I have seen BECMI weapon mastery adapted into AD&D long before the OSR).