Better Shown Than Told

The OSR is a tiny parasol held aloft by a giant elephant; it's too small of an umbrella term to cover everything. That's been obvious for a long time. 

Members of the OSR are divided between those who most enjoy fantasy role-playing games and those who best like fantasy adventure games—or "classic" adventure games (CAGs) to avoid an unfortunate acronym. 

Whenever a B/X player asks the difference between an RPG and a CAG, I always say the same thing: 

It is better experienced than explained; better shown than told.

I could spend months waxing and waning about all the subtle nuances contrasting the play-styles, but the reader's time would be better spent playing and running games. (Here's a perfect Discord server to do just that: OSR Pick-Up Games.)

That said, there are differences in the play-styles beyond a preference for a rules-light B/X or the more complete rules set of AD&D. 

While the Eight Mantras of OSR Gaming apply to role-playing games, not adventure games, the fourth mantra remains true: 

4. The OSR is a mindset, not a rules set.

The mindset of those at the table is perhaps the most noticeable difference between the RPG and CAG styles. 

More specifically, CAG games are of a mindset where "rulings not rules" isn't safe to assume, and I think that's a core difference any member of the OSR who enjoys RPGs will appreciate knowing ahead of time when playing CAGs.

Both are fun, interesting, and challenging mindsets, so rather than write about games, I prefer to run and play them. However, for more discussion on the topic, try the Classic Adventure Game server: https://discord.gg/xakPP2V8yB