This study identified robust predictors of expressive skills in academic English as a foreign language. The participants were 92 Korean-speaking learners of English. The field test of the Pearson Test of English Academic was used as a secondary data analysis. Four communicative skills (reading, writing, listening, and speaking) and six enabling linguistic traits (written discourse, oral fluency, grammar, pronunciation, spelling, and vocabulary) were analyzed using writing and speaking skills as dependent variables. The greatest overlapping variance was found in the relationship between reading and writing. When listening and speaking skills were controlled, the unique variance accounted by reading in the prediction of writing became marginal. The results of this study demonstrated that L2 oral language skills became increasingly important to L2 writing outcomes as language proficiency improved over time.