The Great Owl is widely considered, by the animals of the forest (and the region around the Fitzgibbons farm) to be the wisest of all birds, and possibly the oldest.  But only the very brave or very foolish ever venture to his home to seek his counsel.  And yet it is clear that, somehow, he has made some substantial and (given his dining preferences) unusual alliances...because he certainly knows and respects Jonathan Brisby and the rats of NIMH.  Despite his forbidding air and even more forbidding appetite, however, he is not altogether hostile.  Which is a good thing, since his wisdom and advice may be the only thing standing between Mrs. Brisby and the loss of her son.

Quote: "Go to the rats.  In the rose bush.  Go there!  Ask for Nicodemus."

Origins of the character: In Robert C. O'Brien's book "Mrs. Frisby And The Rats Of NIMH", it is not the shrew who encourages Mrs. Frisby to go and see the owl, but Jeremy the young crow.  And he is never described, in the book, as "the Great Owl".  Jeremy tells Mrs. Frisby that "over that way" (nodding in the direction of the deep woods and faraway mountains beyond the Fitzgibbon farm) "about a mile from here there grows a very large beech tree, the biggest tree in the whole forest.  Near the top of the tree there is a hollow in the trunk.  In the hollow lives an owl who is the oldest animal in the woods--some say in the world.  When we don't know what to do, we ask him.  Sometimes he answers our questions, sometimes he doesn't.  It depends on how he feels.  Or as my father used to say--what kind of humor he's in." 

The owl is very old.  And, after providing Mrs. Frisby advice, as he is preparing to leave for his nightly hunt, he laments about the condition of his old beech tree, and that it may fall soon.  And that, if it does fall and he is still alive, he will fall with it. 

In the animated sequel: The Great Owl does not himself make an appearance in the animated sequel.  He is only mentioned several times.  First in the prologue, where the narrator misquotes him, and then at various moments in the movie itself.