How to make a circle the same size as \otimes?

From the package wasysym.

In text mode: \Circle$\otimes$
In math mode: $\Circle\otimes$

Only in math mode: \ocircle.

Without font packages:

\documentclass{article}
\usepackage{pict2e}
\usepackage{xcolor} % just for the example

\makeatletter
\DeclareRobustCommand{\onontimes}{%
  \mathbin{\mathpalette\on@ntimes\relax}%
}
\newcommand{\on@ntimes}[2]{%
  \vcenter{\hbox{%
    \sbox0{\m@th$#1\otimes$}%
    \setlength\unitlength{\wd0}%
    \begin{picture}(1,1)
    \linethickness{0.35pt}
    \put(.5,.5){\circle{.8}}
    \end{picture}%
  }}%
}
\makeatother

\begin{document}

$A\otimes B\onontimes C$

$\otimes$\llap{\color{red}$\onontimes$}
$\scriptstyle\otimes$\llap{\color{red}$\scriptstyle\onontimes$}

$\onontimes$\llap{\color{red}$\otimes$}
$\scriptstyle\onontimes$\llap{\color{red}$\scriptstyle\otimes$}

\end{document}

It's not perfect pixel by pixel, but it should be indistinguishable.

You also can use \ovoid from matha, one of the fonts from mathabx. Here is a code to use only this symbol:

\documentclass{article}

\DeclareFontFamily{U}{matha}{\hyphenchar\font45}
\DeclareFontShape{U}{matha}{m}{n}{
<5>matha5<6>matha6<7>matha7<8>matha8<9>matha9
<10><10.95>matha10
<12><14.4><17.28><20.74><24.88>matha12
}{}
\DeclareSymbolFont{matha}{U}{matha}{m}{n}
\DeclareFontSubstitution{U}{matha}{m}{n}
\DeclareMathSymbol{\ovoid}{\mathbin}{matha}{"6C}

\begin{document}

$ \oplus\enspace \ovoid \enspace \otimes $\

\end{document}