When the output of an OR gate is applied as input to a NOT gate, the combination acts as a NOR gate. To understand why, let's examine the behavior of these logic gates.

An OR gate outputs a logic high (1) when at least one of its inputs is high. Mathematically, this can be expressed as:

$$ Y_{\text{OR}} = A + B $$

Where $ A $ and $ B $ are the inputs, and $ Y_{\text{OR}} $ is the output of the OR gate.

An OR gate followed by a NOT gate inverts this output. The NOT gate, also known as an inverter, changes a high input to a low output, and vice versa. So the output of the NOT gate can be expressed as:

$$ Y_{\text{NOT}} = \overline{Y_{\text{OR}}} = \overline{(A + B)} $$

This is exactly the logic operation performed by a NOR gate. Therefore, this combination of an OR gate followed by a NOT gate behaves as a NOR gate.

So, the correct answer is Option B: NOR gate.