My first choice for an O word would have been Onomatopoeia, but I can see too many people picking that one (and I haven't met anyone yet who hadn't at least heard of the word). Instead I have chosen a word that is pretty hard to say clearly five times fast, namely Orrery:
A mechanical model of the solar system that allows one to move the various planets around at their correct velocities around the sun
Okay, how many of you sat there trying to say it five times? It's not that easy, at least to say it intelligibly. It's a neat word though, and it's a tool I have always thought would be awesome to have in a large scale. Remember the movie The Dark Crystal? I loved that huge model Aughra used for their own solar system. It was so cool looking and I imagined how much fun it'd be to hang out in that room watching it move around.
Any other images this word conjures for anyone?