Implementation of Any Combinational Logic Circuit with a Decoder and some OR Gates
This smells of homework, and anyway finding such things out for yourself is far better for you in the end, so I'll just give you a hint.

Draw the truth table of the n-input 1-output function that you want to make. The 2n outputs of your decoder correspond to the rows in your truth table. You want the result to be a 1 in the rows where you have put a one in the output column. Now how would you connect a many-input OR to achieve that? If have only let's say 2-input OR gates, how could you make an x-input OR?