24 pin to 24 pin is the same wiring no matter how you slice it. There may be a few strands that differ, just connect them the same location on the HP harness side. Chop the end that came with the opus psu to the desired overlapped length, then splice the hp one to it.
But before doing so, take a picture of each connector for reference so that you can wire them back together. Or simply do them one at a time.
You can also save some money by going to a computer shop and ask for a defective psu with a 24 pin molex. Who knows they might just give it to you for FREE if you take the entire case.
When splicing the wires together for best results, use solder and the appropriate heat shrink tubing for the splice. Cut the 24+ wires at various lengths, splicing them one at a time, so that the harness will be uniform in diameter when completed. Splicing all the wires in one spot will look terrible, most especially when done with barrel type crimp connectors.