Find a word OT has used only once.

[Kaitzilla]

M.a.s.t.i.c.a.t.o.r

Nom nom nom

 

[Kyriakos]

There are more such, but obviously they can appear more than once (twice, often) because someone has quoted the post containing them. Such happened with "nosocomially" and (of course) hyperprotopansevastohypertatos.

 

[Saxo Grammaticus]

Attempted to find another four-letter one (we had one on the previous page) and would like to submit this to the rules committee to see if it counts (try searching it for the reason I'm dubious):

l.u.n.k

I am rather impressed by your four-letter singleton. Could not come up with any off the top of my head. Seems I am better-suited to five letters instead: é.c.l.a.t.