I am planning to build an outdoor stone oven. It will be sited on a poured concrete roof about 6 inches thick (that is reinforced). Will the weight of the oven which I estimate at about 300kg be too much for the roof? The oven weight will be supported on two 40 x 6 inch slabs. ( i.e 240 square inch on each side). Thanks

May need to give more details to get a reply.

What is the span of the roof slab between its supports? Where is the reinforcement placed in the existing slab and is it rebars, mesh? If rebars, near bottom of slab, middle, top?
Size of bars? What is the condition of the existing concrete- cracking etc?
The additional pads/slabs you mention are to adjust the height of the oven? The pads will increase the lbs/.sq. in=-psi (kg/sq. cm?), so sitting flat on the roof would be better but maybe not practical. Any method to spread the load to a larger area (treated 4x6's maybe) would reduce the psi. 4x6's would weigh approx 1/4th the weight of an equal volume of concrete.
It would only be a guess on anyone's part from the information given, as to ability to withstand the additional load.

Ask the roof inspection engineer (http://www.universalengineering.net/services/structural-design-engineers/building-inspection/) about this. I think 300kg would be bit heavy. Keep it around 220-260 kg, so that roof can take that much load easily.