Cooled Exhaust Gas Recirculation (EGR) technology provides significant benefits such as better cycle efficiency, knock tolerance and lower NOx/PM emissions. However, EGR dilution also poses challenges in terms of combustion stability, power density and control. Conventional control schemes for EGR engines rely on a differential pressure sensor combined with an orifice flow model to estimate EGR flow rate. While EGR rate is an important quantity, SwRI believes that intake O2 mass fraction is a better indication of EGR capturing quantity as well as “quality” of EGR. SwRI has successfully used intake O2 mass fraction as a controlled state to manage different types of EGR engines – dual loop EGR diesel engines, low pressure loop /dedicated EGR gasoline engines as well as dual fuel engines. Several suppliers are currently developing intake O2 sensor but they typically suffer from limited accuracy, response time and reliability. Also, addition of a new sensor implies increased production costs. EGR technology is evolving in the gasoline sector as one of the necessary technologies for meeting future emissions and fuel economy standards. This paper focuses on design and implementation of a Luenberger-style nonlinear observer for estimating intake O2 mass fraction. The concept is demonstrated with data from a dual loop EGR diesel engine and dedicated EGR gasoline engine A natural extension of the observer is also demonstrated on a non-EGR port injected dual fuel (diesel and natural gas) engine.