Materials for transparent conductors are essential as electrodes e.g. in photovoltaics and flexible or printed organic electronics, as well as for transparent optoelectronic devices and circuits. A candidate to replace indium tin oxide in flat panel display and optoelectronic devices and circuits is the polymer poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS), a hole transport material widely used in organic electronic devices. We have studied the influence of two different additives, silver nanowires (AgNW) and graphene oxide (GO), on the electrical conductivity and optical transmittance using dimethyl sulfoxide (DMSO) as secondary dopant. For AgNW-PEDOT:PSS transparent conducting layers, the values of the figure of merit are significantly increased by DMSO doping. Hybrid GO-PEDOT:PSS films with low dopant concentration (GO 0.03 wt%) in combination with DMSO (5 vol%) exhibit a sheet resistance of 82.8 Ω/sq, a transmission of 87.9% at 550 nm, and an rms roughness of only 5.4 nm ± 0.3 nm. Thus, low concentration doping of PEDOT:PSS with GO together with DMSO provides an interesting alternative to the use of AgNW, e.g. (i) for applications where low layer roughness is required or (ii) for the manufacturing of transparent circuits and thin films by inkjet printing where nozzle clogging by high aspect ratio nanowires needs to be avoided.