Poly(3-octylthiophene), (P3OT) in addition to its electronics properties exhibits a high Kerr coefficient, n2, due to its third order nonlinear dielectric susceptibility. At the wavelength of 1550 nm, this coefficient n2 is one of the highest. So, this material should be suitable to build integrated all optical switching devices. To construct this device, it is necessary to make a single-mode optical waveguide. For the time being, such a P3OT waveguide has never been obtained due to excessive optical losses. In view to produce single-mode waveguide with P3OT as a core, we investigated the different causes of these optical losses in the material and in the guiding structure. We characterized the optical transmission at key steps in its development. First, we emonstrated that the intrinsic polymer absorption is not a limiting factor at 1550 nm, and then we studied the transmission properties of planar (1-D confined light) and channel waveguides (2-D confined light). The results revealed that better transmission properties can be achieved using planar waveguides rather that confined channel waveguides. This article describes the development and the characterization of the guiding structures that enabled us to identify the main origins of optical losses.