Optoacoustic imaging is emerging as a noninvasive imaging modality that can resolve optical contrast through several millimeters to centimeters of tissue with the resolution achieved by ultrasound imaging. More recently, applied at multiple illumination wavelengths, multispectral optoacoustic tomography (MSOT) offered the ability to effectively visualize tissue biomarkers by resolving their distinct spectral signatures. While the imaging potential of the method has been demonstrated, little is known on the sensitivity performance in resolving chromophoric and fluorescent substances, such as optical functional and molecular reporters. Herein the authors investigate the detection capacity and physical limits of tomographic optoacoustic imaging by simulating signals originating from absorbing spheres in tissue-mimicking media. To achieve this, a modified optoacoustic equation is employed to incorporate wavelength-dependent propagation and attenuation of diffuse light and ultrasound. The theoretical predictions are further validated in phantom experiments involving Cy5.5, a common near-infrared fluorescent molecular agent.