Environmentally benign natural agents in the treatment of water are increasingly gaining interest due to their cost effectiveness, low toxicity and inherently renewable characteristics. In this context the aqueous mucilage of Opuntia ficus-indica (OFI), a species of cactus, was utilized as a bio-coagulant for pre-treatment of OSPW (Oil sands process-affected water) for the first time. Coagulation/ flocculation tests were performed both on model turbid water and OSPW to explore the performance of extracted bio-coagulant on different types of water with varying turbidity. Effect of different operating parameters such as initial turbidity, pH, different salts, storage time and dosages was studied. The bio-coagulant performed extremely well with 98% turbidity removal from OSPW at initial pH of 7 and 8 using 1500 mg/I within 60 min. The active coagulant compound in mucilage of OFI was found to be a combination of pectic polysaccharide, non-polysaccharide and the natural electrolytes especially divalent cations such as Ca+2 and Mg+2. Formation of exceptionally thread like large flocs reveals the possibility that mechanism of coagulation by OFI is adsorption facilitated by inter-particle bridging. A comparative study between bio-coagulant and alum was also conducted and it was found that bio-coagulant resulted a compact sludge with less volume compared to alum.