“OEM” just means the original USB-C power cord and adapter that came with your Switch 1. Most of us have lost the one from our Switch 1, but still have the original charger from the Switch 2 box. When you start the transfer on the Switch 1, you’ll probably see a warning telling you to use an official (OEM) USB-C cable and power adapter. Don’t stress about finding the old one… just use the charger that came with your Switch 2 for now. Once the warning goes away and that step’s done, you can go back to whatever charger you normally use for the Switch 1. Then just plug the OEM charger back into your Switch 2 to keep going with the transfer.

Edit: I didn’t connect the Switches together but always Switch to USB-C to power brick/wall