The oscillations of liquid slugs and vapor plugs inside an oscillating heat pipe (OHP) are captured and shown in terms of pressure frequency spectrum. Pressure fluctuation inside a single loop OHP between the evaporative and the condensing sections is analyzed to evaluate thermal performance. De-Ionized (DI) water and Cu/water nanofluid with different mass fractions (0.5 wt%, 1 wt%, 2 wt% and 3 wt%) are studied subjected to the evaporative heat load of 20-120 W. Swirling, bubble creation and bubble growth phenomena are observed inside OHP integrated with circulation and oscillating motion of vapor plug and liquid slug. Furthermore, deposition of nanoparticles is found on the surface of the evaporative section of OHP when charged with nanofluid. Experimental results show that Cu/Water nanofluid containing 2 wt% of Cu nanoparticles facilitates lowest thermal resistance and highest magnitude of pressure fluctuation inside OHP. Maximum 22% efficiency is achieved by Cu/Water nanofluid with 2 wt% of Cu nanoparticles at 80 W of evaporative power input. However, thermal performance is found significantly interrelated with inside pressure fluctuation of OHP. Cu/water nanofluid is found to promote circulation and oscillation of the liquid slug and the vapor plug rather than reinforcing bubble formation and bubble growth inside OHP. (C) 2014 Elsevier Ltd. All rights reserved.