Effective thermal performance of oscillating heat pipe (OHP) is driven by inside pressure distribution. Heat transfer phenomena were reported in terms of pressure and frequency of pressure fluctuation in multi loop OHP charged with aqueous Al 2 O 3 and MWCNTs/Al 2 O 3 nanoparticles. The influences on thermal resistance of aqueous Al 2 O 3 , MWCNTs as well as the hybrid of them in OHP having 3 mm in inner diameter were investigated at 60% filling ratio. Experimental results show that thermal characteristics are significantly inter-related with pressure distribution and strongly depend upon the number of pressure fluctuations with time. Frequency of pressure depends upon the power input in evaporative section. A little inclusion of MWCNTs into aqueous Al 2 O 3 at 60% filling ratio achieves the highest fluctuation frequency and the lowest thermal resistance at any evaporator power input though different nanofluids cause different thermal performances of OHPs.