Re your concern about bracing with OSB and the thickness required.
This is difficult to answer because there are too many unknowns. Alarm bells ring when you say ”in a windy elevation”. I imagine a steep site rising from a cliff looking out to sea in a south westerly direction, the sort of place you could fly a kite made of boiler plate using an anchor chain for a string. Even if we knew how windy, what elevation, what orientation, how exposed is your site and how thick your ply inner lining and shiplap cladding will be you might not get a useful answer!
Bottom line on this is that no one except a structural engineer who from experience of making calculations of wind load on timber framed buildings, or someone with similar knowledge, is likely to hazard giving you an answer. This is because even casual advice given outside a contractual relationship is subject to the law of Tort, i.e. Duty of Care.
Assuming you don’t want to pay a structural engineer and don’t have any friends or relations in that line I suggest you should look and ask around in your locality. Ask if any sheds in similarly exposed locations have been wind damaged. Look at sheds which have stood the test of time and how they were built.
There is basic stuff about wind loading on buildings in Approved Document A of the Building Regulations, including a map of wind speeds etc, which you can download free from
http://www.planningportal.gov.uk/buildingregulations/ . However, to prove a design you need to be familiar with other stuff like BS 6399, BS 5266 and structural calculations.
In my location (middle of UK, open countryside but not severe exposure) I would not bother with OSB on the outside. Hereabouts, shiplap boarding outside and a decent ply lining inside a frame of 47x100 like yours would be more than adequate. You must of course ensure the various elements are adequately tied together to provide horizontal and vertical restraint, to resist wind lift on the roof and possibly the entire building.
A timber house builder could have several reasons to consider OSB on the outside but do they apply to your situation?
Their panels are usually made off site by a subcontractor in a workshop or factory environment using dry materials so no problem with drying of the frame timbers, but they have to get them to site and erect them undamaged.
The panels would need to be weatherproof and robust enough to withstand transport on an open lorry, storage outside on site, handling and erection.
Wind loading on the partially complete structure, before lateral restraint is provided by floors and roof, is a serious concern. Durability pending final cladding could also be a concern.
OSB would be a cheap and effective way to brace the panels and protect the insulation and meet these criteria.
Diagonal bracing between only two adjacent studs will not be very effective. My gut feeling is that an adequately thick sheet of ply or OSB close nailed to the framing will be much more effective than a diagonal brace. When the outer cladding of shiplap is fixed it will provide additional bracing especially if nailed through the battens into the main studs and plates.
Tanalised timber should not be “wet” when sold but it might have a higher moisture content than ideal. I see that your timber should have arrived today so if it is wetter than you would like could you keep it covered somewhere in a stickered stack to dry out before use? If not or you need to press on I would at least leave the inside of the frame exposed for as long as possible before enclosing it.
With respect to drying out, it appears that 11mm, or less, thick OSB may be sufficiently permeable.:
“nominal 11 mm (7/16”) or thinner wall sheathing panels will allow a wall cavity containing green stud lumber and glass fibre insulation to reach an equilibrium moisture content below 19 % in about 60 days.”
http://osbguide.tecotested.com/pdfs/en/tb111.pdf
Note that the same paper says that OSB of 15.5mm or thicker is effectively a vapour barrier so this thickness would not allow any moisture to escape. This info is from Canada and does not necessarily comply with any UK standards or Building Regulations. Another publication on that site states:
“All panels should be spaced 3 mm (1/8”) on all sides to allow for expansion due to moisture changes,”
Sorry I can’t give you a definite answer to your question. I hope you might find something in the above to help you. Ultimately you need to do what you feel comfortable with. Adding OSB to the outside cannot do any harm and as long as it is 11 mm thick or less the advice quoted above suggests it will be permeable enough. I like your photos, looks like a very nice spot for a home office.
