.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 file format can sometimes have fully binary chunks,
.ora saves its 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.