A simultaneous visual and thermal investigation is conducted to study the thermo-hydrodynamics in a silicon-based micro oscillating heat pipe (MOHP). The MOHP has a rectangular channel with a hydraulic diameter of 550 μm (i.e., Bond number Bo = 0.314) and is partially charged with acetone. The fluid flow and temperature oscillation characteristics of the MOHP under different heat loads, filling ratios and inclination angles are examined and analyzed. In addition, the combined flow motion diagrams and thermal resistance contour maps are organized to correlate the fluid flow characteristics and operational parameters with the thermal performance of the MOHP. The results indicate that, a quasi-steady oscillatory fluid motion without complete fluid circulation is observed in the working MOHP, which is characterized by intermittent periods of three elements: stop-over (S), small-oscillation (S-O) and large-oscillation (L-O). The liquid supply to the evaporator under the S-O motion is mainly characterized by the unstable rewetting of the evaporator via the irregular thin evaporating film, while, the L-O motion of fluid could sufficiently supply the liquid to the evaporator in a fashion of a bulk vapor-liquid mixture. The flow patterns in the MOHP mainly include bubbly flow, slug flow, and annular flow, whose emergence and transition are closely related to the typical phase-change phenomena in the MOHP such as nucleation boiling and injection flow. The fluid oscillatory motions, especially the L-O motion, significantly improve the thermal performance of the MOHP. The efficient operating ranges of heat load and filling ratio expand as the inclination angle increases, suggesting gravity still has a non-negligible positive function in the MOHP performance. In addition, the MOHP has the optimum filling ratio of 53% but fails to work at the filling ratio larger than 74%.