DIY Stable Renovation: Need Your Advice and Ideas!

WIAWSNB

Hi SS.
It really depends on the finish you are after, and why - the room's intended use.
If a smooth painted matt finish, as in a normal building, then plasterboard is the default. If you have concerns about it being affected by damp - but you shouldn't have - then there's a moisture-resistant version, tho' I haven't used it.
OSB is good stuff to use, but obviously comes with it's known finish. I built a store for the local yoof club a few years back, and painted it in a mid grey matt emulsion afterwards, and it looks suitably 'industrial' - totally fine as it was inside a larger boat shed. Some trendy Archie's even employ OSB's natural finish for their projects.
Ply, to me, would be the least aesthetically pleasing, unless it had a veneer or was of very decent - pretty flawless - quality. To get one with a flaw-free finish suitable for painting will likely be costly. When painted, it can otherwise look like just a large featureless panel of painted wood. Ie, what it is. 
To me, the only timber surfaces that look good painted are either smooth, or rough. A ply's smoothish-but-i-can-see-the-flaws looks terrible.
I guess, for both OSB and ply, you could overlay a thick lining paper to get a smoother finish for painting, as you suggest, but why go that route?
Another trick is to 'panel' the wall by adding architrave for interest. The framed panels then become a true feature rather than a plain wall of flat timber.
Other options include timber, or painted, cladding. Or the current trend of 'acoustic' wall panels - thin vertical stripes.
Have a good Pinterest for shed/den/garden room interiors. And even a Fb Marketplace gander for 'wall panelling' or similar - you might see something that's of interest.
A couple of points; could you describe the cross- section through your wall, please, so folk can determine if there could be condensation issues building up?
And, provided this shed is built well enough to not leak through the roof or the walls, and it's ventilated correctly - at least a constant trickle - then you should have no damp or mould issues whatsoever, so p'board should be fine.
Eg, if you use it during the day or eve, even with other folk, and you have heating on so it's warm and moist (matron!), when you depart in the evening, just shut off the heating and make sure the windows are all on vent setting or more. You should have no condie on the panes the following morning.

sujsuj

Thanks.

I'm working on an insulated (or semi-insulated) outbuilding that will serve three distinct purposes:

1. Front Extension/Porch area (yet to be built) – this will run the full length of the shed and be used for storage. 

2. First smaller room – intended to be an office space.

3. Larger room – will be used as a pool/games room.

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🏗️ Construction Details So Far:

Roofing (4 layers, from inside out):

• OSB → 25mm Celotex → Polythene → Metal sheet

Planned Wall Construction (from inside out):

• OSB or plywood → 40mm Celotex → External timber cladding
  (Choice between OSB and plywood depends on balancing interior finish vs moisture resistance.)

Flooring:

• Planning to lay either carpet or wooden flooring, over additional insulation.
  (Moisture levels in the space will influence this decision.)

Ceiling:

• A false ceiling is planned:

  • In the two internal rooms: thin plywood

  • In the front extension: timber cladding

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🌡️ Thermal & Environmental Considerations:

I’m aiming to minimise thermal leakage and make the rooms feel as close to a “normal” interior as possible.
However, I do understand that even with good insulation, this space may not reach the same thermal efficiency as a typical house.

Given that, I’m focusing on materials that can withstand moisture and humidity fluctuations. Durability and long-term performance in this semi-controlled environment are key.

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🎨 Interior Finish Preferences:

• I initially wanted a white interior, but due to the rough texture of OSB, I’m now considering mid-grey matt emulsion on the walls for a clean, low-maintenance finish.

• For the ceiling, I’m leaning toward white-painted plywood to keep the space looking bright and neat.

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❓ Questions & Considerations:

• I’m exploring the idea of applying lining paper over OSB to create a smoother paintable surface.

  • Is this a viable option in a space like this, where there may be occasional humidity or temperature changes?

  • Will the lining paper hold up over time under these conditions?

• There's also a significant amount of void space between the roof and the false ceiling, and I’m keen to make the most of it.

  • I’m considering adding loft roll or additional insulation in that space to improve thermal performance.

  • Alternatively, some of the void space could be used for storage, if feasible.

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📌 Summary

I’d appreciate advice on the best material choices for the internal walls and ceiling that are:

• Cost-effective

• Moisture-resistant

• Durable

• And offer a reasonable finish without excessive prep work

Any suggestions on how to optimise interior finishes and make the most of the void space would also be welcome.

WIAWSNB

I cannot really comment on the wall construction regarding moisture traps, but it sounds ok. I'd rather someone who knows for sure comments. Ie, your OSB then Celotex then the outside cladding.
My Q would be, what happens to any air moisture that might - I don't know if it will - percolate through the OSB? Will it condense out behind it when it hits the Celotex foil and can't get further?
Or, will the presence of the Celotex layer simply resist any moisture travel through the OSB in the first place by keeping it 'warm'?
I have to say this sort of thing always made my 🤕 
So, I'm going to skip that bit and just go to 'finish'.
I'd suggest that, if you want 'durable', as you would in say a proper workshop, then ply would be good. OSB too, but with the caveat of the industrial finish; it'll still look so if painted (I know). I also doubt OSB'll successfully take a 'paper' due to its texture - unless you also used a textured paper!
So, assuming you want 'smooth', then the obvious answer to me is plasterboard. This will be fine, unless water actually gets in to it, which it shouldn't.
You are seemingly concerned about it being affected by 'damp', but this shed should not be damp - you ventilate it when not in use, and it'll remain fresh and dry. Like a 'house'.
If you are still not happy, then use moisture resistant p'board, but it ain't necessary. 
P'board will give you the ultimate finish, in the easiest way - use taper-edged stuff. Here's a thought - use insulated p'board and sort out the two layers in one go!
If you use, say, ply, then you'll have the joints to disguise - or make a feature of. Or, paper-line it to cover, but glue the ply butts to prevent any movement that would crack the paper.
Final finish? Obvs paint if you want. Or, I keep getting pestered by Temu and their striped timber-effect self-adhesive paper which 'looks' pretty thick in their adverts.
Or, Pinterest other options - this is a biggie, as it'll set the feel for the whole building.