Probable lipid transporter involved in lipid countertransport between the endoplasmic reticulum and the plasma membrane. Its ability to bind phosphatidylserine, suggests that it specifically exchanges phosphatidylserine with phosphatidylinositol 4-phosphate (PI4P), delivering phosphatidylserine to the plasma membrane in exchange for PI4P (PubMed:23934110) (Probable). Plays a role in negative regulation of lipid biosynthesis (PubMed:19554302). Negatively regulates APOB secretion from hepatocytes (PubMed:19554302, PubMed:22906437). Binds cholesterol and acidic phospholipids (PubMed:22906437). Also binds 25-hydroxycholesterol (PubMed:17428193). Binds phosphatidylserine (PubMed:23934110). Interacts with OSBPL9 (PubMed:22906437). Interacts with DIAPH1 (PubMed:23325789). Associates with microtubules. The C-terminal region binds cholesterol, 25-hydroxysterol and acidic phospholipids and is required for localization to microtubules. The PH domain selectively interacts with phosphatidylinositol-4-phosphate. Polymorphisms are associated with dyslipidemia. Variant Asn-254 is associated with LDL-cholesterol levels in Japanese population (PubMed:20224571). Association with peripheral arterial disease has also been observed (PubMed:20610895). Belongs to the OSBP family.