The effect of viscosity and bubble content on cavitation bubble oscillation is investigated by the fiber-optical sensor based on the beam deflection in combination with theoretical analysis. The maximum bubble radius at each oscillating cycle in different viscous fluid is determined by experiment. It is observed that the maximum bubble radius at each oscillating cycle does not decrease but increases. A reasonable explanation based on experimental results and numerical calculation is given to this anomaly. The results indicate that both the bubble content and the viscosity have obvious influence on the bubble oscillation and it does increase during the bubble oscillating cycle.