The Mk-II VWs use a dynamic oil pressure sensor, with 2-stage warning system. If on initial startup and below 2000rpm the pressure is excessively low, a low-pressure sensor on the end of the head signals the control unit built into the cluster to flash the oil pressure light. At speeds above 2000rpm the circuit can also be activated by a higher-pressure (1.8 bar) switch; if engine speed exceeds 2000rpm and the switch doesn't close to ground (because pressure doesn't stay above 1.8bar) then the warning control unit activates the buzzer as well as flashing the oil light. So, check the high pressure switch, located on the oil filter adapter, and make sure the spade connecter is properly connected. If that's not the problem try grounding the wire. If the buzzer shuts off, either the problem is the sensor or the oil pressure. I assume the oil level is good... However the lower timing belt cover bolts also secure the main seal housing, so if the mechanic broke off (or just didn't reinstall) those bolts, then the car may have dumped a fair bit of oil out and actually isn't getting enough pressure, the remaining oil may be thinning out from excess heat and though there's some pressure it's not over 1.8bar with the engine at speed. And these things will run an amazingly long time without oil pressure (or with the wrong fluid, one Jetta I owned was driven 8 hours straight with Dexron ATF instead of oil, and aside from running hotter than usual, no ill effects, even 200k kms later). If you need more info feel free to email me.
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Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf, 90 740 Rex/Regina