The speed of the RAM is determined by its frequency (higher the better) and the CAS Latency (lower the better), roughly speaking 1866MHz CL13 and 1600MHz CL11 are of similar speed. 1866MHz CL10/11, 2133MHz CL11 are faster options.
Regarding OWC, I think they are very much overpriced. I would not recommend at all.
If you decide to keep the stock memory, then your choice is very limited. The stock memory is 1866MHz / CL13, which is very slow. When you pair it with new RAMs, it is not guaranteed that the two will work at 1866MHz even if what you buy is labeled as 1866MHz. I heard that the crucial 1866MHz / CL13 memory and Kingston HyperX 1866MHz / CL11 memory will actually work with the stock memory at 1866MHz. G.Skill and Corsair will not! Combined with the stock memory, they will work at only 1600MHz.
Since the stock memory is very slow, I think it is actually a better idea to replace them. Generally speaking, most SO-DIMM DDR3L RAMs will work on late 2015 iMac. I suggest that you get some faster RAMs since they are not necessarily more expensive.
Trusted brands for RAMs are, but not limited to:
G.Skill Ripjaws
Kingston Hyper X
Corsair Vengeance
Crucial Ballistix
Price varies over time and resellers, I would say for 8GB, reasonable prices for 1866/CL11, 1866/CL10, and 2133/CL11 are $45, $50, $60, respectively.
Choose 1866MHz CL11/10 or 2133MHz CL11 for your iMac, you would enjoy 2%-5% overall performance gain over the stock RAM.