We make our furniture and pergolas from Oak.
This is all sourced from local sawmills or direct from local woodlands. Most of the oak we use was grown in Oxfordshire or the surrounding counties. Occasionally we are asked to use continental oak where the customer requires timber that is relatively clean and free from characterful knots and grain.

Mostly we use green oak, that is timber that has not been seasoned. This will dry out over time and will shrink across the grain and may develop small surface cracks or occasionally large cracks in bigger pieces of timber. We carefully design our pieces so that these characterful features do not compromise the strength of the work.
  Some of our furniture requires the use of seasoned or part seasoned timber. We normally use oak that has been seasoned in our store and this may be from 12 months to three or more years old. This timber is more stable and less inclined to move around so is good for finer pieces of work.

Oak doesn't need any preservative to keep it good outside. The tannic acid naturally contained in the timber stops it rotting.