The /o in Ruby regex stands for “oh the humanity!”

This is one of the features that Ruby cribbed directly from Perl. The Ruby documentation seems really bad, in particular “interpolation mode” is grievously misleading.

Perl’s documentation is far more clear about the consequences:

o   Compile pattern only once.

[…]

PATTERN may contain variables, which will be interpolated every time the pattern search is evaluated, except for when the delimiter is a single quote. […] Perl will not recompile the pattern unless an interpolated variable that it contains changes. You can force Perl to skip the test and never recompile by adding a /o (which stands for “once”) after the trailing delimiter. Once upon a time, Perl would recompile regular expressions unnecessarily, and this modifier was useful to tell it not to do so, in the interests of speed. But now, the only reasons to use /o are one of:

[reasons]

The bottom line is that using /o is almost never a good idea.

In the days before Perl automatically memoized the compilation of regexes with interpolation, even back in the 1990s, it said,

However, mentioning /o constitutes a promise that you won’t change the variables in the pattern. If you change them, Perl won’t even notice.

Perl 4’s documentation is briefer. It says,

PATTERN may contain references to scalar variables, which will be interpolated
(and the pattern recompiled) every time the pattern search is evaluated. […] If you want such a pattern to be compiled only once, add an “o” after the trailing delimiter. This avoids expensive run-time recompilations, and is useful when the value you are interpolating won’t change over the life of the script.