Comprehensive experimental and computer simulational studies have been carried out to measure the velocity fields in oscillatory flows around combinations of cylinders and also the forces acting on the cylinders in oscillatory flow. Particle Image Velocimetry (PIV) has been used to measure the instantaneous velocity flow fields. Simultaneous measurements have also been made of both inline force and lift force using a transducer installed at the end of one of the cylinders. The strengths of the vortices in the immediate vicinity of the cylinders were calculated. The numerical simulation study used a random vortex method with three overlapping meshes. Forces on the cylinders, together with the corresponding velocity vector fields, were simulated and comparisons made with the experimental results. The combinations of cylinders were (a) a square array of four cylinders with the dimensions chosen so that the array was an approximate scale model of the Heidrun Tension Leg Platform , (b) two cylinders in tandem with different distances between them, (c) a single cylinder. The Keulegan-Carpenter numbers (KC) were 5.8, 10, 13, 20, the Reynolds numbers (Re) were 2500, 4000, 5000, 6500 and 10000 and the viscous parameter (β = R_e/KC) was set at 500. A comparison of the experimental results with the numerical simulations showed good agreement. For the single cylinder the main frequency component of the lift force shifted from the first harmonic, to the second, and then the third harmonic of the oscillatory flow when the KC number and oscillatory velocity increases. For the four cylinder array, in which the distance between two cylinder was greater than 2.5 times the cylinder diameter, the main frequency component of the lift force shifted from the first harmonic to the second as the KC number and oscillatory velocity increased; the third harmonic was relatively small. The strength and fundamental frequency of the force both increased faster than for the single cylinder case.