The Ondioline was capable of creating a wide variety of sounds. Its keyboard had a unique feature: it was suspended on special springs which made it possible to introduce a natural vibrato if the player moved the keyboard (not the entire instrument) from side to side (laterally) with their playing hand. The result was an almost human-like vibrato that lent a wide range of expression to the Ondioline. The keyboard was also pressure-sensitive, and the instrument had a knee volume lever, as well.
A few videos:
From I've Got A Secret an episode of co-inventor of the ondioline Jean Jacques Perrey. (thanks to JJP for posting this on fb)
See also this previous post on the subject.