There's a word for that.

[MMIX]

What about made up but widely used words. Are they words if used often enough?

Thing-a-ma-bob for example.

By the American definition yes they are.  Words are "official" if they are widely used by English Speaking Americans.

By British and Commonwealth definition only if they are included by Oxford College, so British English has an Ecole.

You may be fight about American English but your ideas on British English are a bit shite, really.

"Oxford College"

Oxford College is one of the leading distance education providers in the United Kingdom and internationally, and along with our partners in education, promotes quality home study education world wide.
[snip]
Your qualification from Oxford College will show your respected and exceptional level of education.

If that is a good example of their grasp of syntax and punctuation they are hardly fit to be held up as examplars of English usage. 

@ CB Thingumbob / thingumabob, was not a very good example because when I looked it up in the Oxford English Dictionary the first recorded usage they give is Smollett 1751, The Adventures of Peregrine Pickle "in a laced doublet and thingumbobs at the wrist" but Smollett was Scots so maybe its not a real English word anyway.
I use the 3 volume microtype version that you have to read with a magnifying glass [supplied in the box case] The OED (which has many different versions) modestly describes itself as

the definitive record of the English language.

my emphasis

This doesn't actually mean that a word has to be in the OED to be an "official" English word. Nor does it have to be pronounced in 'Oxford English'  aka Received Pronunciation, BBC English, or 'talking posh', to be a valid form of communication. And what does the Commonwealth have to do with it . . . ?