When it acts as a base, which atom in hydroxylamine accepts a proton?

The oxygen atom accepts the proton. The oxidation number of O is - 2, meaning that there are two unshared electrons in the valence shell; In the ClO - ion, one of these is shared with the Cl - ion, leaving an unshared electron on the oxygen atom, which is what the hydrogen atom shares its electron with, becoming the proton accepted by the O atom.