HDR is for taking pictures, it doesn't do anything once the video's been made. (It increases the detail in dark areas and decreases the exposure in bright areas. You can't do that after the picture's been recorded - there are no details in dark areas to increase, and flare from a bright light is still going to be flare.)
OLED is just the technology of the screen - many other screen types are also good for viewing videos. Any kind of LED screen will give you black, backlighted screens (like LCD) won't, jet blacks will be really dark grey, but not black. But for watching a movie, you're not really paying attention to the screen quality. Watching a terrible movie on an OLED screen is worse than watching a great movie on an LCD screen.