In this paper, the results of olive oil mill wastewater (OOMW) using physicochemical pre-treatment and Fenton and Fenton-like processes are presented. On the other hand, acute toxicities of raw, physicochemical pre-treated, and Fenton and Fenton-like oxidations applied samples of OOMW on activated sludge microorganisms using respiration inhibition test (ISO 8192) are presented. Chemical pre-treatment (acid cracking and coagulation-flocculation) positively affected the biodegradability and inhibition on activated sludge was considerably removed (> 67% COD and > 72% total-phenol removal). Fenton and Fenton-like processes showed high COD (> 80%) and total-phenol (> 85%) removal performance on evaluated effluents. Inhibitory effect of Fenton-like reagents applied samples on activated sludge mixture was considerably removed. In addition to the toxicity, total-phenol and COD removal efficiencies of applied processes, their associated operating costs were also determined in this paper. (C) 2009 Elsevier B.V. All rights reserved.