The Ryukyuan languages are a family of (at least) five endangered languages spoken in the Ryūkyū Islands, an archipelago stretched between Kyūshū and Taiwan and naturally delimited by the Kuroshio current. They form a sister branch to Japanese, and both derive from a common ancestor, Proto-Japonic (PJ). Ryukyuan can be divided into a Northern branch that includes Amami and Okinawan, and a Southern branch comprising Miyako, Yaeyama and Yonaguni. Traditionally, Japanese historical linguistics has been virtually synonymous with philology, and the reconstruction of PJ has thus exclusively relied on the evidence from the Old Japanese (OJ) texts of the 8th c. CE. Comparative data from the Ryukyuan languages is nevertheless of great importance for this topic, though this importance is still too often underestimated. Based on data I collected in the field and from existing sources, I adduce new comparative evidence from several Ryukyuan languages that add further support to the reconstruction of six vowels in Proto-Japonic, and I propose to reconstruct a new diphthong, namely *oi.