Letter O with vertical line: operator null

Question

I wan to know how this letter O with a vertical line for operator null in maths. Thanks.

Answer 1

\mathds{O} from dsfont package.

Easy to find on Detexify:

\documentclass{article}
\usepackage{dsfont}
\begin{document}
\[
\mathds{O} 
\]
\end{document}