OSR Spell disruption and damage immunity

It's interesting that it isn't taking damage that breaks concentration, magic missile and melf's acid arrow were always great for disrupting enemy mages in the the old crpg games so I assume that was how people were playing it when it came out. I had read about failing saving throws causing disruption but hadn't found the relevant passages.

I'm going to have to have a think about this so that I know how to adjudicate it when the situation comes up, I'm hoping to run a game in the new year (either OSE or 2e) so this is something that is likely to come up. It's also kind of interesting to still be wondering about these things long after the game (not including the various OSR titles) has gone out of print.

I think I like the idea that someone with stoneskin being hit with a sword completely blocks it and allows them to continue casting, I feel like I might otherwise stick to damage disrupting spellcasting in addition to being struck by a weapon, though that means making a saving throw against a fireball spell would still disrupt your spellcast even though only a failed save would according to the rules. I'm thinking of trawling through sage advice after looking through the core rulebooks.

Took a while for me to find (I should have gone straight to the index), but this is what the Rules Cyclopedia has to say about casting spells and being interrupted:

The character must be able to gesture and speak normally to cast a spell. While casting a spell, the character must remain in one place and concentrate. The character cannot cast spells while walking or running, rowing a boat or poli.ng a raft, and so on. If the character is disturbed (i.e .. hit in combat, tackled, etc.) while casting a spell, the spell will be ruined, and will still be "erased" from his mind just as if it had been cast.
- Rules Cyclopedia, Page 32.

Doesn't mention damage but I'd take the bolded to include things like magic missile (unlike the 2e book, it doesn't mention weapons, just being hit). Doesn't mention anything about saving throws so even if you save against that fireball, you still get "hit" and take damage and presumably lose your spell.

I think this is one of those "OSR" referee questions.

No need for hermeneutics - especially from a handbook for a different game (and different from the one OSE is a retro-clone of). What I mean is question seems far more a matter of the fiction then digging through several editions of sometimes contradictory rules. One of the key aspects of OSR and Post-OSR design philosophy is acceptance of spaces where rules run out and that they can be filled with ad hoc referee rulings.

In this case (at least how I'd make my rulings) the nature of the caster and weapon immunity should matter. Lets say we have a regular human wizard who has some kind of magic hat, an undead sorcerer king and a giant spell casting dragon.

The wizard would likely be disrupted by a grapple or melee attack but possibly not a missile attack...mostly because one assumes the guy still flinches.

The lich is likely only disrupted by a very large melee attack that can physically impede its casting so grapples and melee strikes doing more than 8HP damage (a lot for a non-magical melee attack).

The dragon ... yeah you aren't disrupting it's spell casting, not even with a magical attack that damages it unless it's significant, say 15+HP.

Alternately one could look at the fictional nature of the defense - basically if it's a magic forcefield or some kind of immateriality/ghostliness then no magical attacks will be completely ignored. If it's just some variety of being really tough then attacks that don't hurt the thing might still stagger and distract it.

All of this requires a referee or designer willing to engage with the fiction of the monster instead of a pile of statistics and players who are willing to observe and react to description rather then memorize monster stat blocks or mechanics and argue about them.