Overview We investigate the geometric origin of inertial mass within the Origin Geometry (OG) framework, in which effective spacetime is modeled as an emergent description of an underlying discrete four-dimensional aperiodic geometric substrate. In this setting, mass is not introduced as a primitive particle attribute, a phenomenological coupling, or a fitted parameter. Instead, we propose that inertial resistance arises as a structural response of the geometric substrate to metric deformation. The Pre-Dynamical Nature of Inertia The central claim is deliberately pre-dynamical. An excitation is inertial when its existence, propagation, or stabilization requires persistent distortion of inter-site geometric relations across a finite region of the underlying lattice. Metric deformations modify edge lengths, local distances, effective volumes, or higher-order connectivity relations. Such deformations require elastic participation of the substrate. By contrast, approximately isometric, orientation-preserving, or phase-like motions preserve local metric relations and therefore remain energetically transparent in the ideal geometric limit. Volumetric Participation and Mass Hierarchy Within this framework, mass is identified not with motion itself, but with the minimal geometric cost required to sustain metric deformation. Volumetric participation becomes the primary structural source of large inertial resistance: excitations that engage four-dimensional bulk deformation are expected to be heavy in the geometric sense, whereas excitations that remain boundary-supported, phase-like, or approximately isometric are expected to possess much lower geometric inertia. Scope and Limitations This Part does not compute the masses of known particles. It does not identify fermions or bosons. It does not introduce gauge dynamics, Higgs-like mechanisms, or phenomenological fitting. It establishes a geometric precondition for mass: inertial behavior becomes meaningful only when the substrate must participate metrically in the excitation. The resulting definition provides the conceptual foundation for later investigations of mass hierarchy, boundary lightness, bulk participation, and effective low-energy mass terms.