Interesting question. There is no official documentation about that. I suppose it's not here to be connected to SharePoint OnPremises.
I did some research:
Pressed F12 in SharePoint Online to get the URL of OfficeOnline-Server in O365. Then tried to communicate directly via Browser with "/hosting/discovery": Got a useful response
Connected this OOS to my SP2013 OnPremises Environment
Finally i tried to use OOS in my DocumentLibrary. It doesn't work because OOS in O365 cannot download the document from my OnPremises library. Screen stays grey.
I am not able to test the result if SharePoint OnPrem is available via Internet. Might work. But i am very sure it's not intended by MS to be consumed via OnPremises. So it could break at any time.
Installing a local OOS-Server is neither time nor resource intensive. You should consider that!