Any expression or series of gates of only ORs has the non-decreasing property, changing an input from 0 to 1 can never change the result from 1 to 0. So you can

Any expression or series of gates of only ORs has the non-decreasing property, changing an input from 0 to 1 can never change the result from 1 to 0. So you can't implement any gate that needs that using ORs. Similarly, any collection of XOR gates is simply a summation mod 2.