So I have all the parts I need and I was wondering if I could use an old HDD from an OEM computer. I have a copy of windows 7 and the question is, can I wipe the old HDD and install the new windows 7 on it?
Certainly you can provided there's an appropriate IDE or SATA connector and power connector for it on your new build, and of course provided the drive's capacity is large enough for Windows 7 plus a generous amount of free space on the same partition so Windows can "breathe" and run properly.
The easiest way to wipe the old Windows off it is during the Windows 7 Setup which can delete the old Windows partition & create a new one prior to starting the installation. You can deal with any other old partitions (if there are any) using Disk Management when Windows 7 is up & running.