An oscillating positive expiratory pressure apparatus having a housing defining a chamber, a chamber inlet, a chamber outlet, a deformable restrictor member positioned in an exhalation flow path between the chamber inlet and the chamber outlet, and an oscillation member disposed within the chamber. With such layout of an optical system, scan loci of the servo beam corresponding to respective scan lines become closer to parallel. The microbubble emitter emits bubbles with an average diameter of less than 1 mm.