The reaction of OH with NO2 is central to atmospheric chemistry, and its dynamics can be constrained by studying the kinetics of isotopically labeled (OH)-O-18 with NO2. This labeling opens an isotopic scrambling pathway in the reaction coordinate for nitric acid formation, providing experimental constraints on the high-pressure behavior of the reaction with data obtained at low pressures. This reaction, however, is complicated by the presence of a second product isomer, peroxynitrous acid (HOONO), which does not have a scrambling pathway. We present data for the reaction of (OH)-O-18 with NO2 at room temperature between 4 and 200 Torr. The reaction is rapid and independent of pressure. We also locate the H-atom isomerization transition state and show that the isomerization rate constant is at least an order of magnitude faster than adduct dissociation. These results allow us to accurately constrain the formation rate constant of HONO2, which is a factor of 5 slower than the observed OH removal rate constant at high pressure. We conclude that the difference is due to HOONO formation. Our conclusion is consistent with recent theoretical predictions of this branching, and also provides the only self-consistent reconciliation of the high-pressure data with the remainder of the experimental data set.