O2 sensor reads exhaust gases to adjust the mixture for lean and rich. MAF (mass air flow) measures airflow coming into the engine so the ecm can confensate for load conditions. it's a wire that heats up to about 200 degrees celsius, and the ecm monitors how much current it takes to keep the wire at a constant temp. more airflow, the cooler the wire goes and the more current it takes to keep the wire hot etc.