Questions about 1st Edition AD&D

[Premier]

In terms of micro-management versus rules-lite freeflowing, which end of the spectrum would be closer to 1st Edition?

1st edition AD&D specifically... as far as combat is concerned, it depends on which optional rules are in play. With the more complicated optional initiative rules that use weapon speeds, etc. AND using the weapon vs. armour modifier table AND paying attention to minimum space required to wield a weapon, it would get pretty fiddly and slow, though I guess still faster than the infamous "single combat encounter lasting 4 hours" that sometimes crops up in 3E. Without all these extra things - and most of them I've never actually seen used -, it's really, really quick compared to d20.

The main differences are that A, everyone and everything has fewer hitpoints and no things like healing surges that just drag out the fight, and B, a lot fewer variables to keep track of. You don't have a variety of feats, so you don't have to keep track of when they give you certain bonuses or options. It's really quick.

Due to OSRIC using Open Gaming Content, is there anything in the rulebooks (Classes, spells, etc.) which is not in OSRIC?

OSRIC has everything that's in the 1E AD&D Player's Handbook. AFAIK it does NOT have the extra classes, weapons, subraces and spells from the Unearthed Arcana, but I haven't actually checked.

That's assuming you're interested strictly in 1E AD&D. 2E AD&D also had a whole slew of subraces and kits available in its splatbook series, plus some more things in other modules. None of these are in OSRIC.

How much does the power level jump from low to high levels?

Let's use 3rd Edition for comparison: at low levels, it was possible to die in one shot from a lucky critical from an orc's falchion; at high levels, the game was borderline godlike, with spellcasters creating their own pocket planes and demon lords and ancient dragons as opponents. It's like the transition from Law & Order to Superman.

What would 1st Edition be like in the transition from low to high levels?

I was never all that much interested in the high -teen levels of AD&D, so I'm not really qualified to answer that question. But in general, low levels would see you getting gutted by a lucky orc, and really high levels could potentially see you battling demigods and the like - but if you wanted to have any chance of survival, let alone victory, a single such fight would involve a huge amount of planning and preparation.