Option Tree integration of Custom CSS

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.