Oxysterol-binding phosphatidylserine transferase; localizes to ER-PM contact sites through interaction with lipid scramblase Ist2p, resulting in a rapid, directed and sustained phosphatidylserine exchange from the ER to the PM driven by PI4P; family members have overlapping, redundant functions in sterol metabolism and are collectively essential for viability; overexpression extends lifespan by promoting vacuolar fusion; OSH6 has a paralog, OSH7, that arose from the whole genome duplication