Of some reason, Webster 1913 says tells me to look here on ost, but has nothing to offer. This is the missing write-up:
A kiln to dry hops or malt; a cockle.
The oast is an oven to dry hops and malt, ingredients for brewing beer, and thus is an important tool in the brewery industry. The drying process of hops are lighter than that of grain; the hops are left in oast houses over the course of half a day, bringing the moisture level of the freshly harvested hop cones from about 80 percent down to below 10%.