Bombastic means ‘inflated’ or ‘grandiose’ or other words which imply a lack of substantial meaning. Whereas ‘O Fortuna’ makes you quake in your boots at the shear mind boggling randomness of all human life. It’s about the wheel of fate - one minute you’re down and the next you’re the commander of all you survey. And vice versa. It’s LIFE! It’s all that you feel when you’re alive expressed in a couple of chord changes.
Having said all this, I’ve often wondered whether it was just a random turn of the wheel of fate that made this music acceptable to us in the first place. It was being performed in late 30s Germany, and other parts of the piece feel quite ‘Springtime for … Germaneee’ (I mean the music and singing, not the words). Were there similar composers who are not performed now because they failed to distance themselves from the politics of the time as well as Orff did?