Untested cause I'm in a hurry. Oneliner: grep sub { (1 == grep defined, @_) or ($_[0] ne $_[1]) }->(values @{$h->{$_}}['O','C']), keys %$h; More readable:

my $chk = sub { (1 == grep defined, @_) or ($_[0] ne $_[1]) };
grep $chk->(values @{$h->{$_}}['O','C']), keys %$h;
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Not sure how much shorter it is, if at all, didn't have time to count. (Oh my, stress.)

Makeshifts last the longest.