We have 108 names of Vishnu which all starts with OM(AUM). But we call it Hari (and then) OM.
When we say "BhurbhuvasuvarOm iti digbandaha" during Puja/Japa for creating a protective circle around us, then also the (Vyahtri) portion "Bhurbhuvasuvar" comes before Om.
For example, from this page:
sauḥ – aiṁ – klīṁ – hrīṁ – śrīṁ astrāya phaṭ (open up the left palm and strike it three times with index and middle fingers of the right hand)
bhūrbhuvassuvaromiti digbandhaḥ| (using right hand thumb and middle fingers and click clockwise around head)
So, HarihiOm is not the only such case.
Also, starting a recital with Harihiom is not a universal standard it may be practiced by only some Sampradayas. So, the practice is Sampradaya specific.
Who is Hari?
It is another name for Lord Vishnu just like Hara is another name for Lord Shiva.