How to implement a macro for normal ordering of operators?

I am looking to typeset the normal ordering symbols :O: commonly used in quantum field theory in a way that makes them easily discernible from regular colons, especially the 'defined as' symbol :=. Has anybody come across a good LaTeX implementation for such a command which always adds the right amount of white space before and after each :?

Update

In my context, egreg's solution \newcommand{\normord}[1]{:\mathrel{#1}:} appears as follows:

A (rather inelegant) combination of Bernard's and egreg's answer \newcommand{\normord}[1]{{:}\!\mathrel{#1}\!{:}} produces

which I don't think looks great but might the best output I achieved yet.