This study evaluated the psychometric properties of an observational rating system for assessing preschoolers' peer entry and play skills: Observed Peer Play in Unfamiliar Settings (OPPUS). Participants were 84 preschoolers at risk for psychopathology. Reliability and concurrent validity are reported. The 30-min paradigm yielded reliable indexes of engaging and disruptive behavior. Adequate interrater reliability and stability were obtained with minimally trained observers. OPPUS scores were related in expected ways to independent observations of disruptive and engaging behavior during parent-child interactions. OPPUS scores were associated with parent-rated self-control and internalizing behaviors. OPPUS scores were associated with parent ratings of play skills for children with school experience and assertiveness for children without school experience. The OPPUS is a useful, brief method for assessing peer entry and play behaviors in preschoolers.