OEM Apple ram from Late-2015 27" 5K iMac: 8GB (2 x 4GB DDR3) 1867mhz PC3-14900. Also compatible with 2012-2014 iMacs at 1600mhz, and 2016 OR early-2017 iMacs (PC3L / DDR3L).
RAM upgrade instructions:
https://www.ifixit.com/Guide/iMac+Intel+27-Inch+Retina+5K+Display+RAM+Replacement/30533