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¹²O has been observed in the ¹⁶O(α, ⁸He) reaction at E_α = 117.4 MeV (KE78C) and in the ¹²C(π⁺, π^-) reaction at E_π = 164 MeV (BL83E; see for angular distribution) and 180 MeV (BU80B). The mass excess of ¹²O is 32.10 ± 0.12 MeV (KE78C), 32.059 ± 0.048 MeV (BU80B): we adopt 32.065 ± 0.045 MeV. ¹²O is thus unstable to decay into ¹⁰C + 2p by 1.79 MeV and into ¹¹N* + p by 0.45 MeV [note that ¹¹N* is probably not the ground state of ¹¹N and that is very broad: see ¹¹N].

The width of the ground state is ~ 400 ± 250 keV. The diproton branching ratio of ¹²O_g.s. is estimated to be 60 ± 30% (KE78C). The d-coefficient in the IMME for A = 12 is calculated to be 0 ± 11 keV (KE78C), 4.4 ± 7.2 keV (BU80B). The first T = 2 state in ¹²N should occur at E_x = 12.29 ± 0.02 MeV (KE78C). There is some indication of an excited state of ¹²O at E_x = 1.0 ± 0.1 MeV, which would imply an appreciable downward shift from the position of the analog first excited state in ¹²Be (KE78C). See also (TR80D, SE81I) and (WA81D, NG82, AN83D; theor.).