GBP/JPY is now moving within impulse wave 5 of long term downtrend. Wave 5 started from 128.50 (colored light green in the chart). Within this wave there are A and B waves of smaller degree, with wave B still developing from 124.16 (colored orange red in the chart).
The targets of the upmove are Fibonacci retracements of 128.50-124.16.
Resistances:
- 126.84 = .618 retracement
- 128.50 = 1.0 ret
If the long term downtrend resumes the immediate supports will be Fibonacci expansions off 135.11-129.74-132.30, 132.30-124.87-128.50.
Supports:
- 123.91 = contracted objective point (COP)
- 123.61 = expanded objective point (XOP)

Overbought/Oversold
Assuming that the medium term trend is now down it's preferable to try short positions when the Detrended Oscillator gets above the zero level (current prices) or into the overbought area (50-75 pips above the current prices).
Read more on how to apply Fibonacci studies to calculate price targets.