Can you catch a 61c or 56* or such bus to McKeesport? Most of them have racks. I guess you'd have to take a couple of them.
Just riding there is OK, too, especially on a quiet morning. Google directions there work fine; it will be a hilly ride, though, and requires some road-riding prowess. But Oakland to McKeesport doesn't look much (if any) worse than Library to McKeesport.