CREATE OR REPLACE PROCEDURE "FOO" (p_1  VARCHAR(100),
                                   p_2  VARCHAR(100),
                                   p_opt_1 VARCHAR(100) default null,
                                   p_opt_2 VARCHAR(100) default null) IS
...

BEGIN
  FOO("p1 val", "p2 val", p_opt_2 => "p2 opt val");
END;

begin
      cursor = conn.parse('BEGIN foo(:p_1, :p_2, :p_opt1, p_opt2); END;')
      cursor.bind_param(':p_1', 'some string', String)
      cursor.bind_param(':p_2', 'some other string', String)
      cursor.bind_param(':p_opt_1', nil, String)
      cursor.bind_param(':p_opt2_2', 'some value', String)
      cursor.exec
   rescue Exception => e
....