From A Wiki of Ice and Fire
Oakenshield is an abandoned castle along the Wall belonging to the Night's Watch. Castle Black lies immediately to its west. Next along the Wall to the east is Woodswatch-by-the-Pool.
Oakenshield is also the name of one of the four islands that make up the Shield Islands. It is unknown if the two are related in any way.
A Dance with Dragons
Jon Snow re-garrisons Oakenshield as the seat of Tormund.
References and Notes