It could be a front oxygen sensor. It is possible to directly read the sensor output with an analog voltmeter. You would have to back-probe a connector, either right at the sensor or nearer the PCM. A 1 or 2 VDC scale would be appropriate.
Did you also replace the plug wires when the Opti was changed? If so, were the stock aluminum heat shields re-installed at the plugs? It made a difference on my B-body Impala. The wires/boots need that protection. It takes about a half-hour per side to replace the wires into the original clips and looms, but it is worth the effort.