The Moving Average Convergence Divergence indicator (MACD) is comprised of the difference between a 12 period EMA and a 26 period EMA along with a 9 period EMA of the MACD which is used as a cross over/under trigger for signals. Appel used 12, 26 and 9 days but the MACD can be used on any other time period of data (5 minute, hourly, whatever).
The MACD Oscillator or MACD Histogram is the difference between the indicator and its moving average trigger. The histogram is positive when the MACD line (the blue line) is above its signal line (the orange line) and negative when the MACD line is below its signal line. When the two lines cross each other, the histogram crosses zero.
If you drill down mathematically into the performance of the 12 and 26 EMAs, you'll see that after a sustained directional move in the underlying, if the underlying trades flat, the indicator will reverse direction and head toward zero. This is a false signal.
If the MACD turns up when above zero or down when below zero, the signal cannot be false (caused by old data being removed from the moving average calculation). Whether it's a profitable signal depends on what happens going forward.
While there are times where MACD tests well (trending periods), over the long haul it under performs basic simple moving average crossover systems. You can look at research by Colby & Meyers or you can test this yourself.
As for reliability, Indicators based on moving averages (like the MACD) lag the market. Lag means in late, out late with a higher likelihood of missing some of the move as well as whipsaws in non trending markets.
Don't get caught up in the mumbo jumbo of technical analysis. These indicators are mathematical derivative interpretations of recent historical performance. They predict nothing going forward.
As for why your graphs are changing, I can't help you with that because I don't know what your software is defaulting to when you unselect the oscillator function.