Even after Ieshige's death in 1761, Okitsugu was trusted by the 10th shogun, Ieharu TOKUGAWA, Ieshige's heir; he was promoted quickly, given an additional fief of 5,000 koku of rice, promoted from an officer in charge of general affairs and accounts (goyonin) to a shogunate chamberlain (sobayonin), advanced to the junior fourth rank, became the lord of Sagara Castle with a fief of 20,000 koku, and was promoted to a grand chamberlain (jiju) and a senior councilor (roju).
宝暦11年（1761年）、家重が死去した後も、世子の第10代将軍徳川家治の信任は厚く、破竹の勢いで昇進し、明和4年（1767年）にはさらに5,000石加増、御用人から側用人へと出世し従四位に進み2万石の相良城主となって、明和6年（1769年）には侍従にあがり老中格になる。