This study explores the ontological perspective in analyzing students' problem-solving related to mechanical energy conservation. The research involves 18 high school students whose explanations were qualitatively analyzed using an extended ontological framework. Initially, students predominantly employed matter-based predicates and formula-centered representations in the first-step problem-solving. However, in the second step, there was a transition towards process-based predicative expressions and representations, such as reformulating, proportional expressions, symbols, and tables, alongside continued use of matter-based expressions. This phase reflects the students' conceptual shift toward including the "process" category in their understanding of energy concepts. In the third step, most students demonstrated process-based predicative expressions and representations, emphasizing concepts like "conversion" and "transfer." This transition indicates a shift from the ontological category of "matter" to "process," evident in both predicates and representation formats. The findings suggest that understanding students' concepts through an ontological lens not only enhances teaching and learning physics but also contributes to the integration of ontological concepts into science education.