Windows 7 installation DVDs are bootable. In my experience, even the OEM DVDs I've seen are bootable. So if your DVD is not bootable, then I would guess it is a good possibility it is related to the need for the OPK. An OPK is generally used by corporations to do identical installations on numerous computers, but it is not something I have experience using.
It is possible your BIOS simply does not try to access the DVD first in its' boot order. If the hard drive is accessed first then that controls boot, not the DVD. So I would check your BIOS and make sure the OEM DVD is the first thing accessed. If the DVD is first and it definitely is not bootable then I would seek advice from the store that sold you the DVD.
Another option would be to download a matching version (is your key for Home Premium, Professional, Ultimate?) of
W7-SP1(U) and burn that to a DVD using software that understands about ISO images, like
IMGBurn. W7-SP1, if burned properly, is bootable. Then you format the drive and install from the W7-SP1 DVD you burned but use the product key that came with your OEM DVD.