This term became popular in the twentieth century when it was applied to mines where the ground surface was removed and the coal removed without shafts or galleries. It is actually on record though from the early 1700s when the verb ‘to cast’ still retained the meaning of ‘to dig’, as when throwing up a bank or earthwork.
The osier was a type of willow and it had tough pliant branches which once had a variety of uses, especially in basket-work: the word came to be applied more generally to willows and to the branches themselves.
Of uncertain origin, but probably high quality iron from Sweden. Later it came to be used more generally for iron from a number of countries, imported via the Baltic.