\mathbb O
$\mathbb O$

\mathcal O
$\mathcal O$

\mathscr O
$\mathscr O$

\mathrm O
$\mathrm O$

\mathfrak O
$\mathfrak O$

\mathbf O
$\mathbf O$

\mathop O
$\mathop O$

\mathit O
$\mathit O$

\mathllap O
$\mathllap O$

\texttt O
$\texttt O$

\mathclap O
$\mathclap O$

O
$O$