This chapter presents the OMP-A scale (Observing Mentalization in Psychotherapy with Adolescents), an observation measure designed to assess mentalization in the therapeutic context with adolescents. The scale meets the need for an instrument to investigate processes in psychotherapy, integrating both verbal and non-verbal communication and examining mentalization as an interactive and contextual phenomenon. The OMP-A assesses the mentalization of patients and therapists through six key dimensions: Focus on Mental States, Opacity, Contingent Communication, Focus on the Present Moment, Dynamic Nature of Mental States, and Causality of Mental States. These dimensions allow for the analysis of both explicit and implicit aspects of therapeutic interactions, offering a more nuanced understanding of the psychotherapeutic process. The chapter includes detailed clinical vignettes to illustrate the practical application of the scale, demonstrating how its dimensions capture significant moments of therapeutic interaction. In addition, research results are presented to exemplify its usefulness in clinical psychology research. The OMP-A emerges as a groundbreaking tool for advancing the study of psychotherapeutic processes, with potential applications even for everyday clinical practice and psychotherapy training.