According to the OED the term has different spellings and its origin remains obscure. 


***Oojah, oojahslang*** (Also oojar, ujah) 
***of uncertain origin.***

> - A substitute expression used to indicate vaguely a thing of which the speaker cannot at the moment recall the name, or which he does not care to specify precisely; a ‘what-you-may-call it’, gadget. 
>
> - ***1917 W. Muir Observations of Orderly xiv. 229*** ‘Oojah’, anything. > - ***1925 Fraser & Gibbons Soldier & Sailor Words 215*** Oojah (also Ooja-ka-pivi), a substitute expression for anything the name of which a speaker cannot momentarily think of, e.g. ‘Pass me that h-m, h-m, oojah-ka-pivi, will you?’ 
> - ***1930 Wodehouse Very Good, Jeeves! i. 25***  ‘All you have to do,’ I said, ‘is to carry on here for a few weeks more, and everything will be oojah-cum-spiff.

(OED) 



According to the following source the most used form is ***oojah***. 

 [oojah][1] or [oojiboo][2] or oojamaflip:

> - noun informal used when one cannot think of or does not wish to use the name of something.
>
>*ORIGIN early 20th cent.: of unknown origin.*


[Ngram][4]: oojah. 



  [1]: http://www.seadict.com/en/en/oojah
  [2]: http://www.thefreedictionary.com/oojiboo
  [3]: http://it.urbandictionary.com/define.php?term=Oojah
  [4]: https://books.google.com/ngrams/graph?content=oojah,%20oojiboo,%20oojamaflip&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1;,oojah;,c0