When an engine with computer emissions control is first started up, it's in "open loop" mode, where the engine runs on quite a pre-set, rich air-fuel mixture. Once it begins to warm up (including the oxygen sensor(s)), it goes into "closed loop" mode where the air/fuel mixture is set by input from that sensor, and goes from running rich to stoichiometric (14.7 to 1).
Oxygen sensor - Wikipedia, the free encyclopedia
The faster your engine goes into "closed loop" after start-up, the better for emissions & fuel economy.