We studied clean and oxygen-covered surfaces of unreconstructed and reconstructed Pt(110) by density functional theory (DFT) calculations and used these data in thermodynamic considerations to establish the stabilities of these surfaces as a function of the oxygen surface coverage. The clean Pt(110) prefers to reconstruct into a (1 × n) missing-row structure with n = 2–4. The surface free energies of the three reconstructed surfaces are very similar within the accuracy of our calculations. Upon oxygen adsorption, the c(2 × 2) with 0.5 monolayer (ML) coverage on the unreconstructed surface is equally stable as the 0.5 ML coverage on the Pt(110)-(1 × 2) reconstructed surface. There is no clear transition between (1 × 1) and (1 × 2). With increasing oxygen pressure, the fully oxygen-covered (1 ML) Pt(110)-(1 × 2) becomes the most stable structure. We assume that this structure is relevant in the onset of the formation of bulk Pt-oxide. Compared to Au, we found that the Pt(110)-(1 × 2) surface is very stable even under very positive electro potential, and the (1 × 3) structure is not stabilized by impurities (e.g., oxygen).