Protonated species are known to play a key role for ion-molecule reactions in gas phase interstellar chemistry. As hydroxylamine (H2NOH) has never been observed as an interstellar molecule, a detailed theoretical investigation of the protonation of H2NOH is carried out at high level of quantum chemical theories (CCSD(T) and DFT-B3LYP). As protonation may occur directly by reaction with H+ or mediated by H3+, both processes are investigated on the nitrogen and the oxygen sites of hydroxylamine. The present results show that the N-protonated form is more stable than the O-protonated one and that the protonation initiated by H3+ is by far less exothermic than the other one. A particular attention is paid to the intramolecular rearrangement leading from H3NOH+ to H2NOH2+ which involves a highly energetic transition state exhibiting proton bridged between N and O sites. As this barrier is too high to be easily overcome in the interstellar medium, an alternative process mediated by H2 and involving a bridged H3+ as a transition state is considered. The calculations show that the corresponding activation energy is significantly lowered.