Intensity-intensity correlation of a scalar optical beam carrying orbital angular momentum (OAM) across multiple modes is decomposed into intermodal contributions, thereby linking it, within the framework of the Hanbury Brown-Twiss effect, to the underlying modal coherence structure. Upon filtering the spiral phase dependence, the intensity-intensity correlations are governed by OAM-mode coherence and anisotropy reflecting classical entanglement between OAM and statistical properties of light. These results extend intensity interferometry to structured light fields and provide direct access to modal coherence properties without phase-sensitive measurements.