In old age, Otsugu was engaged in improvement of Kannon-ji Temple (Imakumano) which is famous for Kukai and the foundation of Horin-ji Temple (later, Sennyu-ji Temple) and these temples were completed in the generation of Otsugu's second son, FUJIWARA no Harutsu.
晩年は空海ゆかりの観音寺（今熊野）の整備とその隣接地における法輪寺（後の泉涌寺）創建に携わり、次男の藤原春津の代に完成している。