Well, I’ve just answered this question myself.
I removed the calls to UTF8::utf8_strtolower in Bibtex_osbib.php in the formatTitle() function:
//$newString .= $start ? UTF8::utf8_ucfirst( UTF8::utf8_strtolower($v[0]) ) : UTF8::utf8_strtolower($v[0]);
Probably hacky, but it works? (And shouldn’t break anything…)