Langmuir films of poly(o-anisidine) conducting polymer in its undoped (emeraldine base) and doped (emeraldine salt) forms have been obtained by the proper selection of subphase. pi/A isotherms,quartz crystal microbalance, Brewster angle microscopic, and UV-vis-near IR spectroscopic measurements provided evidence concerning the effects of HCl doping on the polymer molecular organization as well as film properties. The analysis underlines that the presence of dopant induces structural and conformational changes on the polymer structure from the emeraldine base to emeraldine salt, so causing a different arrangement of the molecules at the air/water interface. Moreover, the emeraldine base form of poly(o-anisidine) Langmuir films evidenced less compressibility, as suggested by the presence of aggregates at high surface pressures, and a lower collapse surface pressure with respect to the emeraldine salt one. The investigation of the Langmuir film formation by the Brewster angle microscopy made it possible to support the previous experimental results and to image directly two (2D)-three (3D) dimensional transformations, which occurred by overcompressing beyond the limiting densities of close-packed 2D films, namely, collapse pressure. The recorded optical spectra revealed the development of polarons and bipolarons due to inclusion of the dopant in the polymer backbone and also provided an estimation of the poly(o-anisidine) bandgap.