Om or Aum was a constructed word in the later scripture and made sacred to indicate the first meaningful sound human being produced in the process of development of language and the place where it evolved.
Most ancient form of Aryan language is available to us as Sumerian language and Rig Vedic language; a simple observation would reveal that both are captivated by the use of 'U'/'Oo' in word formation. However, Rig Veda representing the earlier stage of language development, uses 'U' a single letter as a word like many other 's', 't' etc. Here 'U' means 'that one' whose name was not evolved yet, if the sound of 'U' is short 'that one' is nearby and if the sound is long 'that one' is at far distant place.
Ancient Indian sages knowing this phenomenon and refraining from giving any particular name for 'that one' who is behind the all creation, used this letter/word and they believed that human life began from water or 'am', so 'U'/'Oo' + 'am' = Om/Aum. In Rig Veda 'am' is earlier term for water than 'ap'.