PNG's do not have the necessary bit depth to function as an HDRI lighting source.
If you look at this video you can see the full dynamic range of a properly created HDRI Image : ( I queued the time that you need! ) You see how the image doesn't fade like a normal JPG does. That's what makes it work as a scene light correctly.
If you lit your scene with an incorrectly constructed HDRI image or a JPG / PNG file it will appear very dark and muddy.
I'm not familiar with Octane specifically. However with most GPU based renderers like Keyshot & Marmoset ( I believe Octane is a similar program ) you just load the HDRI image in and it goes. It's pretty much load & run.