I might opine that OSE Advanced, rather than being closer to 1st edition AD&D, replicates AD&D content/ideas (that is, expanded classes, spells, magic items, and monsters), but mechanically converts them to the simpler, somewhat less powerful B/X rules framework. It also adds a few mechanical options from AD&D (split race & class, and spell acquisition, for two examples), again, in simplified and clarified forms.
Which IS, for the record, the way a lot of folks who stated with B/X, Holmes Basic, or BECMI but couldn't quite parse the AD&D rulebooks actually played AD&D. Kind of kludging them together and simplifying rather than figuring out (say) the intricacies of AD&D initiative rules and so forth.