The only real proper word I know in Croatian for “cloud” is “oblak.” I find the Belarussian and Ukrainian words very interesting. My guess is that “Chmara” is related to “Mrak” or “mračiti se” which is darkness and a verb for dimming. Logically it makes sense since clouds ‘dim’ sunlight. A “Chmara” is literally an object that dims or darkens which is perfectly applicable to a cloud. I’m no linguist though.
the -Ob or -O prefix is used commonly in Croatian. Materially traversal object action, surrounding and spreading around the object. Bringing the enforcement action, to expose the process, execute action on the surface or on the top of which, irrespective of whether there is a main verb. So yes, you’re right about the prefix in addition to the prefix typically bringing an all-encompassing meaning to a regular noun or a complete action to a verb.
Again I’m no linguist.