The Galaxy S8 has a striking design compared to the OnePlus 5.
There's no competition here. The combination of the invisible home button, curved display edges, rounded display corners, taller aspect ratio, and ultra-narrow borders around its display makes the S8 a strikingly beautiful smartphone that also more modern than the utilitarian design of the OnePlus 5.
The Galaxy S8 has a sharper display.
You can set the S8's display to a 1440p resolution, which is sharper than the OnePlus 5's 1080p resolution.
The Galaxy S8 is water-resistant; the OnePlus 5 is not.
Samsung is keen to boast about the S8's IP 68 water resistance, which allows it to survive down to a depth of 1.5 meters (nearly five feet) for up to half an hour.