MACD Is an Oscillator, But...
We can say MACD is an oscillator, as it oscillates around zero. But unlike most other oscillators, MACD has no (explicit) overbought and oversold areas. It also doesn't have a percentage or standardized scale (like 0 to 100 or -100 to 100).
MACD is measured in the same units as the underlying market's price (dollars, euros) and theoretically there are no limits on the values MACD can reach, as the two exponential moving averages can in theory move infinitely far away from one another. In reality MACD tends to come back to zero relatively soon.