List of countable ordinals


 * \(\omega\), 1st transfinite ordinal
 * \(\omega+1\), 1st transfinite successor ordinal
 * \(\omega2\), 2nd transfinite limit ordinal
 * \(\omega^3\), of
 * \(\omega^\omega\), PTO of, RCA0undefinedand WKL0
 * \(\varepsilon_0=\psi_0(\Omega)=\psi_0(\psi_1(0))\) with respect to Buchholz's ordinal collapsing function, PTO of PA and ACA0
 * \(\varepsilon_{\varepsilon_0}\), PTO of ACA
 * \(\zeta_0=\psi_0(\Omega^2)=\psi_0(\psi_1(\psi_1(0)))\) with respect to Buchholz's ordinal collapsing function, also called Cantor's ordinal
 * \(\eta_0=\varphi(3,0)\) with respect to Veblen's function
 * Feferman-Schütte ordinal \(\varphi(1,0,0)=\Gamma_0=\psi_0(\psi_1(\psi_1(\psi_1(0))))\) with respect to Buchholz's ordinal collapsing function, PTO of ATR0
 * Ackermann ordinal \(\varphi(1,0,0,0)=\psi_0(\psi_1(\psi_1(\psi_1(0)+\psi_1(0))))\) with respect to Buchholz's ordinal collapsing function
 * Small Veblen ordinal \(\psi_0(\psi_1(\psi_1(\psi_1(\psi_0(0)))))\) with respect to Buchholz's ordinal collapsing function, PTO of \(\text{ACA}_0+\Pi_2^1-\text{BI}\)
 * Large Veblen ordinal \(\psi_0(\psi_1(\psi_1(\psi_1(\psi_1(0)))))\) with respect to Buchholz's ordinal collapsing function
 * Bachmann-Howard ordinal \(\psi_0(\psi_2(0))\) with respect to Buchholz's ordinal collapsing function, PTO of KP set theory + axiom of infinity
 * \(\psi_0(\Omega_\omega)\) with respect to Buchholz's ordinal collapsing function, PTO of \(\Pi_1^1-\text{CA}_0\)
 * Takeuti-Feferman-Buchholz ordinal \(\psi_0(\varepsilon_{\Omega_\omega + 1})\) with respect to Buchholz's ordinal collapsing function, PTO of
 * \(\psi_0(\Omega_{\Omega_{\cdot_{\cdot_{\cdot}}}})\) with respect to Extended Buchholz's ordinal collapsing function, the countable limit of Extended Buchholz's ordinal collapsing function
 * \(\psi_{\Omega}(\varepsilon_{\text{I}+1})\) with respect to Buchholz's ordinal collapsing function based on the least weakly inaccessible cardinal, PTO of KPi
 * \(\psi_{\Omega}(\psi_{\chi_{\varepsilon_{M+1 }} (0)}(0))\) with respect to Rathjen's ordinal collapsing function based on the least weakly Mahlo cardinal, PTO of KPM
 * \(\Psi^0_\Omega(\varepsilon_{K+1})\) with respect to Rathjen's ordinal collapsing function based on a weakly compact cardinal, greater than or equal to the PTO of KP + \(\Pi_3\) reflection
 * \(\psi_\Omega(\varepsilon_{\mathbb{K}+1})\) with respect to Arai's ordinal collapsing function using the least \(\Pi_N\)-reflecting ordinal \(\mathbb{K}\) for \(N\ge 3\), PTO of KP with a \(\Pi_N\)-reflecting universe under ZF + V = L.
 * Limit of Taranovsky's C function (conjecturally larger than PTO of Z2 )
 * PTO of Z2
 * PTO of ZFC
 * Omega one chess \(\omega_1^{\mathfrak{Ch}}\)
 * Omega one of 3D chess \(\omega_1^{\mathfrak{Ch}_3}\)
 * Admissible ordinals, ordinals \(\alpha\) such that \(L_\alpha\) is a model of KP
 * Church-Kleene ordinal \(\omega_1^{\text{CK}}\), first nonrecursive ordinal and first admissible ordinal>\(\omega\)
 * Recursively inaccessible ordinals
 * Recursively Mahlo ordinals
 * Reflecting ordinals
 * Stable ordinals
 * Infinite time Turing machine ordinals
 * Supremum of all writable ordinals \(\lambda\)
 * Supremum of all clockable ordinals \(\gamma\)
 * Supremum of all eventually writable ordinals \(\zeta\)
 * Supremum of all accidentally writable ordinals \(\Sigma\)