What kind of Barcode Reader USB can work on El Capitan / iMac
A USB bar code reader should just work out of the box.

I am personally using a Symbol (aka Motorola, aka Zebra) bar code reader.  Upon plugging it into my iMac running OS X El Capitan (10.11.6), it just shows up; no drivers were needed for the install.





This is the exact model bar code reader I am using for this example:  Symbol USB Bar Code Scanner PN# LS2208




I have been using this (and another Symbol scanner, but I no longer have it) since I got this iMac with Mavericks (10.9.x) and I never need to install drivers for either of the scanners.

Doing a scan of a simple UPC bar code sends the characters to whatever application I am using (in this case, Notes)