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, intake O2 mass fraction may be 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 several types of EGR engines - dual loop EGR diesel engines, low pressure loop /dedicated EGR (D-EGR) gasoline engines as well as dual fuel engines. Several suppliers are currently developing intake O2 sensors 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 describes the design and implementation of a Luenberger-style nonlinear observer for estimating intake O2 mass fraction. The observer formulation is explained and demonstrated using the Dedicated-EGR (D-EGR) engine topology. Application of the concept on a dual loop EGR diesel engine is presented. A natural extension of the concept is also demonstrated on a non-EGR fumigated dual-fuel (diesel and natural gas) engine.