There are 2 kinds of 'hard drive' you're likely to find in modern PCs.
HDD (hard disk drive) which is the kind of hard drive you're probably familiar with.
And SSD (solid state drive) which is a newer kind of drive that is faster, but more expensive.
In the past, PCs would often ship with just a single, large HDD. Now it's common for PCs to have a small SSD for Windows, programs, and maybe a game or two. And possibly also a large HDD for mass storage. The idea being you get the speed of the SSD to make Windows fast, while not spending too much on SSD storage. 240gb SSD + 1TB HDD has been a common combo.
Also some modern games can be 100GB in size. Not all, but some.
So, as the above have said, it's possible you are 1) installing very large modern games and/or 2) installing them onto the SSD, which is usually smaller than the big HDDs you might be used to.
If you can only see one drive in My Computer: While it's common for pre-built PCs to have both the SSD and the HDD, sometimes the HDD is not formatted so Windows cannot see it. Or sometimes a cable wiggled loose in transit so it's physically not connected. Either is super easy to fix yourself.
Can you link to the store/product page you bought the HP Omen off so we can see what it is supposed to have inside it?