Last week I got the OA3 as I thought its strengths/weakness would nicely compliment the strenghts/weaknesses of my Pocket 2. I was totally right on that.  One thing I'm grappling with, though.  Doing some testing I determined HDR adds significant appeal to image quality, especially on faces.  However, it seems to output H.265 only.  My computer renders H.264 just fine but absolutely abhors H.265.  What would normally be a 20 minute render becomes a "start it before bedtime and hope it's done in the morning" ordeal.  
So here's my total rookie question:  Is it possible to shoot using HDR and output an H.264 file?         

Under Video Compression options in Settings it says:

"Only videos in HEVC format available in following settings:
1. HDR mode
2. D-Cinelike 10-bit
3. 4K (4:3) and supported FPS"
...and so forth.

So the answer is no, you cannot record HDR in H264 on the Action 3. You can convert it, however. HandBreak is a great tool that allows you to set various compression parameters, including speed, and can convert to H264 prolly faster than in other programs.

https://handbrake.fr/

As osmonauta wrote, it's not possible to record HDR in H.264 with the Action 3.

Addidtiona information: It's a codec problem. H.264 (AVC) originally only was designed for 8-bit video. It only supports HDR in the uncommon High 10 profile. So by transcoding from H.265 (HVEC) to H.264 you may loose color information when using an encoder like Handbrake that does not support that profile. I don't know how great the difference would be or if it is clearly perceptible. Or if your computer is fast enough to do the transcoding on an acceptible amount of time...
I have a kind of old computer but it manages trancoding of H.265. Just real time editing is a bit bumpy.

I dunno much about the technical details but it seems like HandBrake does support HDR10 in H264.

Documentation:
https://handbrake.fr/docs/en/1.6.0/technical/hdr10.html

osmonauta Posted at 4-15 12:06
I dunno much about the technical details but it seems like HandBrake does support HDR10 in H264.

Documentation:

Sorry, you are absolutely right. I did not find the option at once because it is not in the H.264 profiles but needs to be selected as encoder itself (strange UI decision). Thanks for the information.

I still don't know how widely it is supported by players, though...

Much thanks for the replies.  I had never heard of HandBrake but I just downloaded it and will be checking it out.  Again, THANKS to all!

Hi there. Thanks for reaching out. As mentioned in the previous comments above, only H.265/HEVC is supported in HDR mode. We will help you forward this feedback to our related team for attention. Significant suggestions or requests will be implemented after the evaluation of the DJI Engineers. For the latest updates on DJI, please stay tuned to our DJI official website at (www.dji.com). Thanks for your understanding and continued support.