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We present a zero magnetic field muon spin rotation study ofα-O₂ (antiferromagnetic phase of solid oxygen) in the temperature range of 10–24 K. Static magnetic order has been observed below theα-β transition temperatureB _αβ=23.8 K. The temperature dependence of the muon precession frequency exhibits behavior characteristic of a two-dimensional Heisenberg spin-1 system with the anisotropy parameterα ∼ 10⁻² quite similar to that of antiferromagnetic phase of the high-temperature superconductor parent compounds. A unique local field at the muon site has been determined to beB ₀=1.27(5) kG at low temperatures.