Construction of OR gate through universal NAND gate

In this gate we get a high output when even on of the input is high. A low output is obtained only if all inputs are low. This operation is signified with A+B and is also called union of A and B

Making of an OR using NAND Gate

The implementation of OR gate using NAND is done by using 3 NAND gates as shown in the circuit diagram.

Z=A+B