Olive mill wastewater (OMW) utilization for microalgae cultivation has been investigated in different studies, in which it is generally strongly pre-treated before use. Here a common pre-treatment method (active carbon) has been carried out for OMW, and its influence on Tetradesmus obliquus growth (generally known as Scenedesmus obliquus) and phenol and sugar removal has been compared with untreated OMW. Before to carry out test on OMW, nitrate concentration in the media was optimized, finding a modified BG11 media more adequate for our experimental aims. T. obliquus used in this work showed a nitrogen requirement of about 4 pg/cell in exponential growth, for a protein content of 34%. Active carbon treatment reduced 90% phenol content and 50% sugar content in OMW. Addition of 9% (v/v) pre-treated OMW to the medium, did not affect microalgae duplication during test. In contrast a cell duplication inhibition was observed after 4-6 days for untreated OMW added at 9% (v/v). Lower final phenol concentration was reached in test with pre-treated OMW (10 mg/L), while no difference was observed in final sugar concentration comparing the two different OMW. Instead higher phenol removal during cultivation was obtained for test with untreated OMW (240 mg/L), indicating the necessity of further tests to better understand mechanism involved in their degradation. A process with active carbon pre-treatment looks more adequate for mixotrophic growth and pollutant removal as main goals, while untreated OMW looks more promising for heterotrophic cultivation.