Was going to say exactly this. In terms of being a system, Dolmenwood is essentially b/x with a few house rules. But clearly it is meant to be run as a long-form campaign, with PCs traversing an extremely dense map, pursuing plot threads that are spread across that map, and engaging with complex factions. People are already running rather extended campaigns in this setting.
I'd argue that what makes a Dolmenwood campaign more 'OSR' and less 2e/AP style play is a group that understands that there are consequences for actions taken in game and that failure is on the table. The 'story' is not a pre-defined set of encounters (encounters in which die rolls are fudged to keep characters alive). The story emerges from what the characters do, how they engage with the factions, who they align with, who they alienate, where they go. It's sandbox play, and actually there is nothing stopping one from running 5e in that style either.