How to add flags to regular expression pattern tests?

How to add flags to regular expression pattern tests in MySQL?

Seemingly the pattern isn't case insensitive for some unicode characters as in:

SELECT  UPPER('ö') REGEXP 'Ö' TrueResult,'ö' REGEXP 'Ö' FalseResult;

Returns:

TrueResult  FalseResult    1           0

Whereas:

SELECT  UPPER('o') REGEXP 'O' TrueResult,'o' REGEXP 'O' FalseResult;

returns:

TrueResult  FalseResult    1           1

So for example, how to write the equivalent of:

SELECT UPPER('ö') REGEXP 'Ö';

with regexp flags instead?