The surface quality of steel depends on initial solidification at the meniscus during continuous casting. A computational thermal-fluid model has been developed to simulate the complex transient behavior of the slag layer between the oscillating mold wall, the slag rim, the slag/liquid steel interface, and the solidifying steel shell. It includes transient heat transfer, multi-phase fluid flow, solidification of the slag and steel, and movement of the mold during an oscillation cycle. The model is validated with transient temperature measurements from a “mold simulator” lab experiment and with plant measurements of slag consumption. During an oscillation cycle, thermocouple variations depend on their location relative to the meniscus and the rate of heat flow increases greatly when steel overflows the meniscus. The model reveals new insights into the phenomena which govern initial solidification, oscillation mark formation, and surface defects in this process.