The LaTeX command for big O (๐ช) is \mathcal{O} (big-O notation (asymptotic upper bound)).
\mathcal{O}Writing a paper? The table generator and the BibTeX cleaner are one click away, no account.
Write \mathcal{O} inside math mode, that is between $โฆ$ for inline math or in a \[ โฆ \] or equation block for display math. It is part of core LaTeX math, so no extra package is required.
In pseudocode or running text, wrap \mathcal{O} in $...$ so it keeps math spacing and fonts. With unicode-math under XeLaTeX or LuaLaTeX, typing ๐ช in a formula gives the same symbol.
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\lfloor x \rfloor\lceil x \rceil\top\bot\models\mathsf{NP}\mathsf{P}\Omega\Thetao\oplus\land\lor\negComputer science symbols in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \mathcal{O} in math mode; it needs no extra package. For example, $\mathcal{O}$.
None: \mathcal{O} is built into LaTeX, so no extra package is required.