Ion-molecule reactions between clusters of H2/D2 and O2 in liquid helium nanodroplets were initiated by electron-induced ionization (at 70 eV). Reaction products were detected by mass spectrometry and can be explained by a primary reaction channel involving proton transfer from H3(+) or H3(+)(H2)n clusters and their deuterated equivalents. Very little HO2(+) is seen from the reaction of H3(+) with O2, which is attributed to an efficient secondary reaction between HO2(+) and H2. On the other hand HO4(+) is the most abundant product from the reaction of H3(+) with oxygen dimer, (O2)2. The experimental data suggest that HO4(+) is a particularly stable ion and this is consistent with recent theoretical studies of this ion.