I am a bit at a loss how a you had a shop repair a cylinder head without pre and post diagnostics to give the vehicle back running properly or explaining what was needed to make it so. 
Yes a properly working O2 sensor will lean the mixture out as compared to having it disconnected. Most likely the richer mixture with it disconnected is masking some other fault. Also that richer mixture is bad news for the catalytic converter. Knowing and  willingly operating the vehicle with part of the emission control system disabled may be one of those things you do not want to document. 

Performance and drivability issues usually come down the engine mechanical,  fuel-air delivery, ignition, and exhaust.

1) Compression and leak down test. I would be leery of a valve problem on the other side. Possible problem with rings on side head was repaired on. 
2) Ignition, no more miss fires, timing set properly. Vacuum and centrifugal advance working. 
3) Fuel-air delivery. All injectors delivering same quantity of fuel. Good spray patterns. All cylinders drawing same air volume. No vacuum leaks.
4) Exhaust sound. Cat and muffler not clogged. No leaks. 

Dennis 

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of John Lauterbach
Sent: Thursday, December 22, 2011 12:15 AM
To: vanagon@GERRY.VANAGON.COM
Subject: Can O2-sensor make engine run too lean?

Have been running engine with O2-sensor disconnected for past couple of weeks.  Engine performance okay in other regards with sensor disconnected.  My Son and I put in new O2-sensor this afternoon.  We made sure connections were tight and connector end of cable was not grounded.  Once engine is warmed up, it lacks power when starting from a stoplight.  Nothing much happens when you push down on the accelerator pedal until engine get some speed on it.

Any suggestions on next steps?

John

'84 Vanagon, 1.9L engine