Is there a easy way to convert hierarchical expression to a single expression?

[danielchen26]

'O = NOR(I1), I1 = NOR(I2, I3), I2 = NOR(b), I3 = NOR(I4), I4 = NOR(I5, a), I5 = NOR(c)'

For example, I want the output O to be written in terms of a, b, c. how can I directly translate these parallel hierarchical expression into a single logical function and get truthtable.