The identification of interstellar HOC+ has been confirmed through new observations of this species at 3, 2, and 1.2 mm using the NRAO 12 m telescope. Toward Sgr B2 (OH), the J = 2 --> 1 and J = 3 --> 2 lines of this ion at 179 and 268 GHz were detected for the first time. In addition, the J = 1 --> 0 line of HOC+ at 89 GHz was observed toward several new sources, including Orion-KL. In conjunction with past observations of the J = 1 --> 0 transition toward Sgr B2 (OH), these data clearly establish the presence of this species in the interstellar medium. The column density toward Sgr B2 (OH) is estimated to be Ntot ~ 3 x 1012 cm-2, which corresponds to a fractional abundance relative to H2 of f ~ 10-12. Measurements of HCO+ were also carried out toward Sgr B2 (OH), resulting in an abundance ratio of [HCO+]/[HOC+] ~ 140--360 at gas densities of n(H2) > 105 cm-3. This ratio contradicts past predictions of ion-molecule models by several orders of magnitude and suggests that the reaction HOC+ + H2 --> HCO+ + H2 is not as fast as previously thought.