The principle is obviously the same as when you juggle a flag, you just need to time your actions right (grab the bomb/ball again directly after dropping it in mid-air) and look upwards at the right time and at the right angle. And don't forget to crouch. It's pretty difficult to pull off (even harder with lag) and thus not nearly as useful as the flag jump you can do from the ground floor or simply throwing the objective up there before you jump up yourself. It won't hurt to have it as a trick up your sleeve tho.
Not sure if I ever tested it with an oddball, but it should work the same way.
