How to Upgrade RAM on iMAC 5K Late 2015 - OWC
Maxed out the iMac 5k and maxed out the ram by adding 32 gig ram 1867 mhz from OWC purchased from BH Photo and Video.
If you use anything prior to the late 2015 iMAC 5k then it should 1866 mhz ram. you can technically use slower ram in this late 2015 model but the late 2015 model takes advantage of this 1867 ddr3