.ora, or the Open Raster format, is an interchange format. It was designed to replace *.psd as an interchange format, as the latter isn’t meant for that. Like *.kra it is loosely based on the Open Document structure, thus a zip file with a bunch of xmls and pngs, but where Krita’s internal fileformat can sometimes have fully binary chunks, .ora saves it’s layers as *.png making it fully open and easy to support.

As an interchange format, it can be expected to be heavy and isn’t meant for uploading to the internet.