I posted this in Blender Bugs forum but the issue occurs in stand alone as well.
Problem occurs only when Alpha Channel is enabled. I don't know if this is a bug or expected behavior, but .exr files output from Blender's file exporter do not have the issue (one can enabled Alpha Channel and still get a normal AO pass in Octane Blender). The issue only occurs when outputting the .exr files using Octane's export implementation.
More info in my Octane Blender post:
viewtopic.php?f=114&t=74498