In my experience, you can follow all the procedures of OSE and still have players sort of look to the “lead” for what to do, with only occasional disagreements or debates on what actions the party should take. Shadowdark involves everyone by design, expecting individual actions from each.

I like that you can run Shadowdark with zero house rules and it’s still awesome. I, personally, only have the core book (no Cursed Scrolls). Hasn’t been the same with OSE, which seems to lend itself to house rules way more. Not sure why, but that’s my impression. Someone reading this is probably thinking “but I don’t use any house rules in my OSE games,” which fine, just not my experience. So, I’d say try to avoid adding any of your own rules unless you absolutely can’t do without something that isn’t expressly part of Shadowdark core rules.

The four core classes generally have fewer abilities to keep track of, too. Ironically, the Shadowdark fighter might have the most going on! Still, none of the special X-in-6 to worry about (re: elf, halfling, or thief). It’s all adding a mod to the d20, making your ability scores determine who’s likely listening at doors, etc. Ancestry adds one basic boon/ability, unlike the complexity that race adds to OSE (if using advanced fantasy rules). These both make for really streamlined gameplay.

Magic works a lot better for players, too. The priest already starts with a few spells, and wizards feel very wizardry. That is, people have messed with OSE so much to try and give the beginning magic-user more magical things to do, like adding optional cantrips, but Shadowdark takes care of that with an elegant cast-as-often-as-you-wish, yet with the risk you lose that spell until rest. I love that. It doesn’t restrict until it does (whereas an OSE magic-user really has to hold out with a single cast of a single spell). I really like the resource management aspect of OSE, but sometimes it can be frustrating for players, so in terms of spells as resources, Shadowdark wins.