Oustiti is an irregular and perhaps a bit archaic spelling of the French ouistiti, sometimes Americanized as wistiti. It refers to a marmoset, coming from onomatopoeia of the monkey's cry. The term was primarily used in English in the early 1800s, before primatology jargon was properly formalized.
It is also used, in French, as the equivalent of the American 'cheese!', used to cue smiles for the camera.
And it is also used, in novels, and perhaps only in novels, to refer to a lockpicking tool used on doors locked with a skeleton key. It is said to be a thin pair of tweezers used to reach through the keyhole from the outside, to turn a key that has been left inserted in the lock on the inside. It seems to have been invented by Hugh Wiley in his story The Eye of Heaven (1938), and then popularized in Georgette Heyer's locked-room mystery novel Envious Casca (1941). It has since been glommed onto by a number of words lists and dictionaries, a smattering of later mystery novels, and a few serious books on crime published years later. It is generally said to have been a term used by international hotel thieves, and perhaps it was; 1930s-era international hotel thieves were not known for etymological documentation, so we may never know.