never* execute code outside of hooks as you are more likely to call a function or use a variable before they were declared and initialized.
your code should be
function css() {
if (ot_get_option('css', false) != false) {
echo'<style>'.ot_get_option('css').'</style>';
}
}
add_action( 'wp_head', 'css', 100 );
*never say never, but this is a rule to follow unless you have a very explicit reason to ignore it.