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Abstract The dominant form of oxygen in cold molecular clouds is gas-phase carbon monoxide (CO) and ice-phase water (H₂O). Yet, in planet-forming disks around young stars, gas-phase CO and H₂O are less abundant relative to their interstellar medium values, and no other major oxygen-carrying molecules have been detected. Some astrochemical models predict that gas-phase molecular oxygen (O₂) should be a major carrier of volatile oxygen in disks. We report a deep search for emission from the isotopologue¹⁶O¹⁸O (N_J= 2₁− 0₁line at 233.946 GHz) in the nearby protoplanetary disk around TW Hya. We used imaging techniques and matched filtering to search for weak emission but do not detect¹⁶O¹⁸O. Based on our results, we calculate upper limits on the gas-phase O₂abundance in TW Hya of (6.4–70) × 10⁻⁷relative to H, which is 2–3 orders of magnitude below solar oxygen abundance. We conclude that gas-phase O₂is not a major oxygen carrier in TW Hya. Two other potential oxygen-carrying molecules, SO and SO₂, were covered in our observations, which we also do not detect. Additionally, we report a serendipitous detection of the C¹⁵NN_J= 2_5/2− 1_3/2hyperfine transitions,F= 3 − 2 andF= 2 − 1, at 219.9 GHz, which we found via matched filtering and confirm through imaging.