Clean surfaces of $p$-type GaP, GaAs, GaSb, and InAs and $n$-type GaAs, AlSb, and GaSb have been prepared by crushing in ultrahigh vacuum (${10}^{\ensuremath{-}9}$ Torr) and measured by the electron-paramagnetic-resonance technique at room temperature and 77\ifmmode^\circ\else\textdegree\fi{}K. A small clean-surface signal was found. When oxygen was adsorbed at 77\ifmmode^\circ\else\textdegree\fi{}K, a new signal was found, due to $\mathrm{O}_{2}^{\ensuremath{-}}$ ions. In the case of AlSb, hyperfine structure was resolved. A complete analysis was carried out without the usual restriction of setting the $\stackrel{\ensuremath{\leftrightarrow}}{\mathrm{A}}$ (hyperfine) and $\stackrel{\ensuremath{\leftrightarrow}}{\mathrm{g}}$ tensors parallel. Parameters were checked by computer simulations of the spectra. About 5% of the wave function of the unpaired electron on the $\mathrm{O}_{2}^{\ensuremath{-}}$ molecule is localized on the Al atom. The unfilled (dangling) orbital of this surface atom is found to be over 90% $p$ type. This provides, apparently, the first experimental determination of a clean-semiconductor-surface wave function. A model of the (110) surface of a III-V compound semiconductor is proposed. The group-V atom dangling bond contains an electron pair in a $p$ orbital, while the group-III atom dangling bond is a largely empty $p$ orbital. In-surface bonds are largely $s{p}^{2}$. Parameters for the clean-surface signal on GaAs and GaSb are $g=2.0038\ifmmode\pm\else\textpm\fi{}0.0004$, width 10 \ifmmode\pm\else\textpm\fi{} 1 G, corresponding to about 4 \ifmmode\times\else\texttimes\fi{} ${10}^{11}$ spins ${\mathrm{cm}}^{\ensuremath{-}2}$, reduced by about 30% after exposure to air. For the $\mathrm{O}_{2}^{\ensuremath{-}}$ signal on GaAs ${g}_{\ensuremath{\parallel}}=2.036$, ${g}_{\ensuremath{\perp}}=2.007$, and on AlSb ${g}_{\ensuremath{\parallel}}=2.041$, ${g}_{\ensuremath{\perp}}=2.005$.