Oxidative deamination is the release of ammonium via a conversion of glutamate to alpha-ketoglutarate.
Glutamate dehydrogenase converts glutamate to alpha-ketoglutarate (which is the original acceptor amino acid found in the transamination reaction).
Water provides the oxygen and the hydrogen necessary to oxidize alpha-ketoglutarate and protonate ammonia to ammonium (the non-toxic form of the molecule).
We show ammonia in its charged state (as ammonium) because this is how it typically exists at physiologic pH (rather than as uncharged ammonia).