This study investigates relationships between global oral proficiency ratings and measures of grammatical competence in the acquisition of Korean as a second language. Data were collected on the linguistic abilities of learners at 1+ to 4 on the ILR scale, focusing on perception in phonology, morphology, syntax, lexis, and collocation. The results show that (i) most of the tasks have high internal reliability, (ii) individual accuracy scores correlate strongly with levels on the ILR proficiency scale on most tasks, and (iii) heritage speakers outperform non-heritage speakers at the same high levels of oral proficiency on most tasks. The findings indicate that global proficiency scales like the OPI can be deconstructed using tasks that provide detailed measures of learners'control of linguistic features
