His house and habits
The owl lives in an hollow inside a dying tree. Like all the owls, he cannot see in the light. This is why, he used to come out in the night (dark) for hunting. Mice are among his preys. Nevertheless, he doesn't harm or trap any prey that comes freely in his home. He seems to follow other principes, such as avoid discussing problems with people he cannot see in front of him and he is also thankful and willing to help in return, any person in trouble who have been helful to a bird. Besides, The owl sleeps during the daytime, but he waits awoken until the dusk ends in his house. His house is spacious and clean, but the floor like stalagmites, is extremely rough. He builds himself a nest in his house with twigs and leaves and lined it at the top with feather's wisps. The owl faces the same problem of "moving day" like the Frisbys. While his living house which is inside a dying tree may fall down one day, he doesn't yet cognize a solution.
Scenes, he shows up in
We can see the owl, when Mrs Frisby, needing an advice, goes to his home with Jeremy. Despite she fears at first to be eaten by him, the owl indicates to her a great way to save her whole family, showing to her meanwhile that he has a profound respect to her late husband. In fact, The owl knows the rats of NIMH who living in the rosebush near Fitzgibbon's garden, particularly Justin and Nicodemus. He knows also what they are able to do for instance, moving Frisby' house "in the lee of the stone".
Source : O'Brien, R.C (1971). Mrs. Frisby and the Rats of NIMH. New York: Aladin Paperbacks.