Operational transfer path analysis (OTPA) is a diagnosis method aiming to identify and rank noise transmission paths in dynamic systems. The particularity of the method is to require no preliminary acquisition of a transfer matrix between excitation and response dofs, as it is the case for classical TPA approaches. OTPA is based on the identification of a transmissibility matrix between some input and output responses measured for various operating conditions. The first difficulty of the method concerns the definition of this transmissibility matrix, from either a theoretical or experimental point of view. The second difficulty is the use of this matrix for diagnosis purposes, requiring some assumptions leading to potential misunderstandings. Theoretical aspects of the method are firstly discussed in this work. Secondly, an experimental validation is carried out on an academic test setup, and OTPA results are compared to the classical TPA approach.