This study examines the relationship between second language use outside of class and oral proficiency development. It first identifies out-of-class activities of international graduate students in the U.S. and the average time spent speaking English in those out-of-class activities. Interviews and student self-measurements of time spent speaking English each day were used to investigate the types and quantities of out-of-class activities. In addition, two sets of student oral proficiency test scores were collected. Correlation analysis is used to find out the relationship of the variables between the most salient out-of-class activities and oral proficiency gains. The findings indicate that second language use outside of class is important for international graduate students to improve their oral proficiency. This is especially true with regularized interaction such as talking at work and the average time spent speaking in English a day outside of class. This study suggests that learners of English in an ESL environment should be encouraged to take part in out-of-class activities in addition to English use in the classroom in order for them to improve their oral proficiency.