Newly designed oxide glass–ceramic electrolyte of Li2.9B0.9S0.1O3.1 with high Li+ ion conductivity and low melting property was prepared by mechanical milling and subsequent heat treatment at 290 °C. This material showed 1.4 × 10−5 S cm−1 at room temperature and excellent deformation properties to obtain powder-compressed pellets with low interfacial resistance like in the case of sulfide solid electrolytes. The glass–ceramic exhibited favorable mechanical properties to form favorable solid–solid contacts in solid-state batteries by pressing without high temperature heat treatments. All-solid-state In/LiCoO2 cells using these oxide glass–ceramic electrolytes operated as secondary batteries at room temperature.