Sulfonated poly(1,3,4-oxadiazole-2,5-diyl-1,4-phenyleneoxy-1,4-phenylene), poly(1,3,4-oxadiazole-2,5-diyl-10,10-dioxophenoxathiine-2,8-diyl) and their copolymers were one-pot synthesized in fuming sulfuric acid with the use of 4,4 0-oxydibenzoic acid and hydrazine sulfate as initial reagents. These copolymers were non-fusible and did not dissolve in individual liquids except sulfuric acid. However, it was possible to achieve their unlimited solubility using a mixed solvent that contained dimethyl sulfoxide, formamide and water. In dilute and semi-dilute solutions, the copolymers behaved like polyelectrolytes, while in concentrated solutions they formed gels; moreover, in the case of high polymer content, the gels were in a liquid crystalline state. The degree of sulfonation, temperature and water content in the mixed solvent influenced the state of copolymer solutions, their viscosity and viscoelasticity. The finding of the mixed solvent made it possible to form polymer films whose strength and heat resistance reached 33 MPa and 470°C, respectively. The ionic conductivity of the films under direct current conditions was 0.001-0.01 ∼S cm-1 for sodium cations and 0.3-1 mS cm-1 for protons.