||Oh, Young-il. (2022). An optimality-theoretic analysis of open syllable lengthening in Middle English. The Linguistic Association of Korea Journal, 30(1), 73-92. This study provides an analysis of Open Syllable Lengthening in Middle English (ME) within the framework of Optimality Theory. Unlike previous constraint-based studies (Sohn, 2005a, 2005b, among others), this study introduces constraint reranking and shows a gradual process of Middle English Open Syllable Lengthening (MEOSL). The data dealt with are ME nāme /na:mə/, ME god /god/, ME alfe /alfə/, and ME fugeles /fugeləs/, and the constraints used are Max-Seg, FtBin, Dep-μ, and NonFin. The constraint hierarchy for pre-MEOSL is Max-Seg 》 FtBin 》 Dep-μ 》 NonFin, and the hierarchy for post-MEOSL is Max-Seg 》 FtBin 》 NonFin 》 Dep-μ. For MEOSL in progress, the unranking process occurs between Dep-μ and NonFin, and the hierarchy is Max-Seg 》 FtBin 》 Dep-μ, NonFin. With these hierarchies, this paper offers a consistent explanation for both examples of and exceptions to MEOSL and properly represents a gradual process of diachronic sound change.