The OEM plug and the aftermarket head unit do not match, but there are a few different ways to deal with that situation. The easiest way is to buy an adapter harness. If you find a harness that's designed specifically for your head unit and vehicle, you can just plug it in and go. You may also be able to find a harness that you can wire in to the pigtail that came with your new head unit.
The other option is to cut the OEM harness and wire the aftermarket pigtail directly into it. If you choose to go that route, you can use either crimp connectors or solder.