This article extends Optimality Theoretic studies to the research on second language tone phonology. Specifically, this work analyses the acquisition of identical tone sequences in Mandarin Chinese by adult speakers of three non-tonal languages: English, Japanese and Korean. This study finds that the learners prefer not to use identical lexical tones on adjacent syllables, especially the contour tone sequences. It is argued that the Obligatory Contour Principle (OCP) was playing a role in shaping the second language Chinese tonal phonology even though it was not learned from these speakers' native languages, nor found widely applied in the target language. The acquisition order of tone pairs suggests an interacting effect of the OCP and the Tonal Markedness Scale. This study presents a constraint-based analysis and proposes a four-stage path of OCP sub-constraint re-ranking to account for the error patterns found in the phonological experiment.