Well, it includes natural healing rules, for one. Sorry. Still salty about that.
OSE isn't BECM condensed into one book like the RC. OSE is a B/X retroclone (Classic Fantasy) expanded to include stuff from AD&D (Advanced Fantasy). So if RC is your benchmark, there will be some differences.
I'd say it's way more playable than RC simply because many of the things added over the course of BECMI are not there. Though OSE does have rules for things like domain management, which appeared in the BECMI line. Weapon mastery is gone. General skills are gone. But, in Advanced Fantasy, AD&D's weapon proficiencies and secondary skills are presented as optional rules, each of these subsystem takes one page. The level cap is 14 instead of 36. They have options for both THAC0 and the modern ascending armor class and attack bonus. This is baked into every class chart and monster, so there's no looking it up to use either.