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The detonation propagating through the annular channel of an optically accessible nonpremixed rotating detonation engine (RDE) is visualized in this work using OH* chemiluminescence imaging. The fuel and air are injected from separate streams and partially premix in the channel in front of the detonation wave. The OH* chemiluminescence images allow observation of the size and shape of the detonation structure, trailing edge oblique shock wave, and possible presence of deflagration between the fuel fill region and expansion region containing detonated products. The OH* chemiluminescence images are useful for evaluating the effects of the air mass flow rate, equivalence ratio, air injection area, and fuel injection scheme on the detonation structure and its corresponding impact on RDE operation and performance. The detonation increases in height as the air mass flow rate is increased for low flow rates, experiences subtle changes in size and shape for intermediate flow rates, and transitions from one-wave to two-waves as the flow rate is further increased. For fuel lean conditions, the high OH* emissions from the detonation are distributed more broadly in space. For stoichiometric and fuel rich conditions, the high OH* emissions typically are confined to a more narrow region near the detonation wave front. The wave front is more concave with respect to the fuel fill region in front of the detonation as the air injection slot is increased from low to intermediate values.The angle between the wave front and fuel injection surface in front of the detonation becomes more acute as the air injection slot is further increased. Reducing the number of fuel injection holes has significant effects on the detonation structure including transition from one-wave to two-waves. The waves typically co-rotate with the detonations propagating in the same azimuthal direction for most conditions in which two-waves are established in the channel. Counter-rotating waves with the detonations propagating in the opposite azimuthal direction are observed for some conditions. The observation of two counter-rotating detonation waves demonstrates one occasional effect of non-ideal mixing between the fuel and air in a nonpremixed RDE. The OH* chemiluminescence images are useful for evaluating RDE models and simulations, improving fundamental understanding of the detonation structure in nonpremixed RDEs, and identifying critical design parameters that influence RDE operation and performance.