The Principle of Ontological Preservation (POP) establishes a minimal ontological restriction on the admissible attribution of fundamentality within physical theories, introducing non-trivial constraints on the ontological interpretation and unification of physical descriptions across domains. POP is developed from the requirement that explanatory decomposition cannot coherently proceed indefinitely without unresolved logical regress. Under this restriction, genuinely fundamental constituents in their pure state must correspond to ontologically irreducible elements that are absolutely simple and non-decomposable. However, isolated relations between absolutely simple constituents are not sufficient to account for the differentiated complexity exhibited by physical entities and phenomena. Physical entities currently treated as elementary within physical theory — including the electron and other apparently elementary particles — therefore cannot consistently correspond to genuinely fundamental constituents, but must instead correspond to structurally organized configurations grounded in relations between fundamentally simple constituents. Under POP, the multiplicity of physical properties attributed to such entities implies unresolved structural organization incompatible with ontological primitiveness in the strict sense required by irreducibility. Physics is consequently interpreted as structurally mediated through dynamically organized higher-order entities rather than as direct access to ontologically fundamental constituents themselves, with the multiplicity of physical properties arising from such structural organization rather than from multiple properties intrinsically attributed to isolate entities. Within this framework, the persistent fragmentation between quantum, relativistic, and classical descriptions does not primarily originate from mathematical incompatibility nor from multiple independent ontological foundations, but from the attribution of primitive ontological status to entities whose behavior already presupposes unresolved structural complexity. Quantum discreteness and relativistic invariance are therefore interpreted as reflecting transformations and preserved relations within structurally organized physical entities rather than direct manifestations of ontologically primitive constituents or primitive spacetime ontology itself. POP therefore provides a restrictive framework for identifying the ontological level effectively described by existing physical theories while introducing cross-domain ontological consistency as an admissible condition on the coherent unification of physics. Existing physical formalisms may retain operational validity while remaining ontologically incomplete, requiring convergence toward a common underlying ontological basis. Under POP, ontological requirements acquire a constraining role in delimiting admissible foundations for physical unification while providing a structural basis for the construction of coherent unified frameworks grounded in first ontological principles.