The electronic and the geometrical structures of the polymers poly-1,3,4-oxadiazole and poly-(p-phenylene-1,3,4-oxadiazole) are studied using density functional calculations. These polymers are fully conjugated, having band widths which are comparable to the more common conjugated polymers. They have however a significantly higher ionization potential, which makes them suitable hole blocking materials for organic light emitting diodes. In addition poly-1,3,4-oxadiazole is a convenient electron transport material, due to its high electron affinity. Due to the presence of the phenyl rings, the electron affinity of poly-(p-phenylene-1,3,-oxadiazole) is much lower.