Could be a number of things really. Could be just moisture in the wire boots. Are your wires original? I assume they are. With them moved around during the install they can break inside and cause misfires. Thats why I tell anyone with a 3800, if you want me to change your plugs and you have original wires, buy wires and we'll do both. I refuse to just do plugs w/ original wires. I know of many people who simple pulled off there plug wires to change just plugs, when the put the wires back on they misfired like crazy. The wires get brittle and weak with age.
Also too you vac lines are old and they could of been disurbed while the install. The hoses or fittings could be cracked. A can of carb cleaner will tell ya. As for the oil everywhere? Most likely the "Quailtified/Certified ASE Mechanic" (my ass!) didn't clean up his mess when doing the install. He just poped the cover off (doubt he even cleaned the surfaces), poped a new gasket in and that was it. You can't trust anyone to do anything anymore. As for seeing a gasket hanging out, I can't see it in the pic but it wouldn't surprise me, alot of mechanics half ass things. The 3800 Valve cover gaskets i've seen, the colors are usually grey or orange and its an o-ring like material. If the oil is low or reappearing, you gasket may not be sealed. Speaking of oil, you may want to check it or top it off, better yet, change it.
As for the O2 sensor, that probably ain't helping things, get that replaced and don't replace it with a POS Bosch crap. Get an AC/Delco or Denso.
Could be one of many things, it most likely simple and stupid and I wouldn't be surprised if its a mechanic screw up...
