ACtually there are two modes of operation on EFI vehicles.
First is "open loop". This is where the ECM, PCM, or whatever you call it, does not read inputs from the O2 sensors; it will receive inputs from the TPS, the coolant sensor, the Crank sensor, and the cam sensor, if it has one. It runs the engine from a pre mapped program.
There is cold start, warm up, and normal op temp in the phases.
Once the engine temp reaches a pre programed range, the PCM will then go into "closed loop" mode where it begins to receive inputs from the O2 sensors. Newer rigs have an upstream O2 sensor and a downstream O2 sensor to more carefully monitor the A/F ratio and help the ECM keep it within range.