Cedar was a wise choice given that it is naturally durable. Especially focus any treatment on the portions of the structure that will be in contact with the ground....you might repaint it with more oil annually or something like that.
One other natural wood-preservation technique is to char the surfaces in a fire until they blacken and begin to "checker" and then quench. The wood is then coated with a layer of comparatively rot-proof charcoal. This treatment supposedly will double the life of an in-ground post of whatever species, as compared with an uncharred post of similar species and size. But your
chicken pen might not be made of pieces thick enough to permit this treatment without simply burning them up....