If the signal from the O2 to the ECM is open or shorted to ground, that O2 will read lean. If it reads lean, more fuel is mistakenly pumped in . Same is true of a bad O2, but since it's new I doubt it's bad.
What A/F ratio gauge??? Where is it hooked in???
Cheap A/F guages use a normal O2 sensor for data. These can be very inaccurate if the mixture is out of range. Wideband O2's (also called a UEGO, or Universal Exhaust Gas Oxygen sensor) stay accurate but are very expensive.
Any high impedance meter will do for an operational check. An osciliscope is needed only to find quirks in a working system.
The O2 should be getting 12vdc to the heater, usually on the two white wires.
The signal to the ECM would be black for + and grey for - .
This signal bounces around with an average of about 0.45 vdc. If the output is zero, there is probably a wiring fault.
