If what you've done is install a non-Dell mainboard, you will need a new Windows license for it - even if you get a copy of Windows, the OEM license key won't activate it. When you call Microsoft, you'll be told you need a new license (and MS isn't selling Windows 7 licenses any longer, as the OS goes end of life in just about a year and a half from now).
Microsoft pulled the Digital River ISO downloads for 7 and 8 and 8.1.
There is nowhere to do this now. Only 10 is available.
There is no such thing as OEM LOCKED.
OEM keys work with the SLP in bios or System Builder DVD's and a LEGIT COA that isn't pirated and copied and sold 1 million times.
In order for OEM System Locked Preinstallation (SLP) to work your new motherboard will need to be a Dell motherboard with a System License Internal Code of 2.1 embedded in it's BIOS. A generic (non-Dell) motherboard will not have the SLIC 2.1 and be ineligible for OEM SLP.