A knock, as of a visitor and not a postman, was heard at my door. “Goodness gracious,” I exclaimed, “why didn’t we sport the oak?” Samuel Butler – “The Way of All Flesh”
"Sporting the oak" refers to the open or closed state of the outer door,
indicating the inhabitant's readiness to be disturbed.
“In my Oxford days, to "sport the/one's oak" meant to close the (outer) door of one's set of rooms in college as a sign that one was engaged and did not want to be disturbed. To bang on a fellow student's door when the "oak was sported" would have been regarded as a gross solecism. The expression "to sport the oak" is attested as far back as the eighteenth century (see OED).” F.W.Langley - Hull University