Doubly ionized oxygen undergoes a rare "forbidden" electronic transition.
Electrons in individual hydrogen atoms flip their intrinsic spin states from being parallel to being antiparallel to that of their parent protons.
Molecular hydrogen undergoes a rotational transition from a higher to a lower energy state.
Electrons in the first excited state of atomic hydrogen make a transition to the ground state.
All of the mechanisms listed here can produce 21-cm emission.