Yes, you do. Unless you have an unused retail (full) version of Windows XP/Vista/7 that you can install, and then upgrade to Windows 8 (upgrading to 8 from an older Windows is currently cheaper than the OEM version).
I am building a new computer and thinking of installing windows 8 onto it do I need the Oem version or not.
You can install whichever version you desire but please be aware of the licensing restrictions imposed by OEM licensed copies, namely that the product will be bound to the make and model of the motherboard on which it is installed.
Given how affordable and stable Windows 8 is, I highly recommend it.
Given the choice between Windows 8 and Windows 7 I would chose Windows 7 every time. However before the end of this year Windows 8 is available for a cheap price and if I had to pay for a operating system I would be inclined to save my money and get Windows 8, which is why Microsoft is offering Windows 8 at a cheap price. Windows 8 can be made tolerable with the addition of Classic Shell which will let you boot up into the Desktop, restore the start menu and enable Aero if told to.
If you buy your operating system next year get Windows 7 not 8.
I pretty much agree with everything said here so far. (harsh apache.) As long as MS is selling Win8 so cheap you might as well use it. You don't need anything like pro, whatever they suggest for home users is fine.