Oscillators are popular and widely used because they are leading indicators that can signal a possible trend change that is yet to start.
This type of indicator oscillates between two limits, above and below a midpoint and its value helps to gauge the strength and momentum of a trend. Oscillators also typically signal if a market is overbought or oversold (meaning price is unjustifiably high or unjustifiably low), which could point to a reversal of the trend.
This could be used to determine when to close open positions.
Oscillators work best in ranging markets because in trending markets they can show overbought or oversold conditions too soon. Common things to look for are a midpoint cross, approaching maximum or minimum value and regular or hidden divergence.
Oscillators are usually plotted with a line or histogram. There are many oscillators such as the Relative Strength Index, the Stochastic Oscillator, the True Strength Index and the Ultimate Oscillator.