Inhibits angiogenesis in the vitreous humor of the eye, and therefore represses neovascularization (By similarity). Binds collagen fibrils (By similarity). May be involved in collagen fiber organization via regulation of other members of the small leucine-rich repeat proteoglycan superfamily (By similarity). Homodimer. Expressed in cartilage and synovial membranes (at protein level) (PubMed:18164633, PubMed:23845380). Expressed in the retina, iris, ligament, skin and fetal liver (at protein level) (PubMed:12019215, PubMed:25136834). Expressed in the retinal pigment epithelium (at protein level) (PubMed:25136834). Expressed in synovial fibroblasts and subchondral bone osteoblasts (PubMed:18164633). O-glycosylated. Proteolytically cleaved by MMP1, MMP2, MMP3, MMP7, MMP8, MMP9, ADAMTS4, and ADAMTS5 (PubMed:23845380). Proteolytically cleaved by MMP13 (By similarity). The degradation of OPTC by proteases may contribute to osteoarthritis pathophysiology (PubMed:23845380). Sulfated on tyrosine residues. Belongs to the small leucine-rich proteoglycan (SLRP) family. SLRP class III subfamily.