The transport properties of ceramic materials strongly depend on oxygen activity, which is tuned by changing the partial oxygen pressure (pO(2)) prior to and during measurement. Within, we describe an instrument for highly stable measurements of Seebeck coefficient and electrical resistivity at temperatures up to 1300 K with controlled oxygen partial pressure. An all platinum construction is used to avoid potential materials instabilities that can cause measurement drift. Two independent heaters are employed to establish a small temperature gradient for Seebeck measurements, while keeping the average temperature constant and avoiding errors associated with pO(2)-induced drifts in thermocouple readings. Oxygen equilibrium is monitored using both an O-2 sensor and the transient behavior of the resistance as a proxy. A pO(2) range of 10(-25)-10(0) atm can be established with appropriate gas mixtures. Seebeck measurements were calibrated against a high purity platinum wire, Pt/Pt-Rh thermocouple wire, and a Bi2Te3 Seebeck coefficient Standard Reference Material. To demonstrate the utility of this instrument for oxide materials we present measurements as a function of pO(2) on a 1 % Nb-doped SrTiO3 single crystal, and show systematic changes in properties consistent with oxygen vacancy defect chemistry. An approximately 11 % increase in power factor over a pO(2) range of 10(-19)-10(-8) atm at 973 K for the donor-doped single crystals is observed.