xcf is GIMP’s native format. It is layered, lossless and has weak compression.
ora is supposed to become the open format for layered raster images.
..a collaborative effort between the GIMP developers and Krita developers..
Unfortunately there are no command-line .ora converters and no .ora viewers. This means that a website or file viewer can’t automatically create previews and an artist will have to open the file in the editor to view it.
There seems to be a lack of xcf-viewers as well but at least there are command-line tools and imagemagick can read it as well.
OpenRaster also has a create wiki page.