The O2 signal should rapidly fluctuate up/down (at least once per second), between 0.1-0.9 V, with the DC component centered around 0.45VDC. Vida should show that. Engine at operating temp, idle. If fluctuates slower, or it's stuck low/high the sensor is bad. You already have the mass air flow, you ...

Your STFT highly positive at idle only points to an air leak. MAF seems to work okay based on the same observation of the fuel trims.