GBP/JPY is developing wave C of long term downtrend - colored light green in the chart. And within this wave there are A, B and C subwaves, with subwave C still developing - colored magenta in the chart. Within this subwave there are A and B waves of still smaller degree. The targets of the upmove are Fibonacci retracements of 130.51-128.45.
Resistances:
- 129.24 = .382 retracement
- 129.48 = .50 ret
- 129.72 = .618 ret
If the price keeps moving down and breaks below 128.45 the supports will be Fibonacci expansions off 139.93-130.26-135.11, 135.11-129.74-132.30, 132.30-128.90-130.51.
Supports:
- 128.41 = contracted objective point (COP)
- 127.11 = objective point (OP)
- 126.93 = OP

Overbought/Oversold
Assuming that the medium term trend is down it's preferable to try short positions when the Detrended Oscillator gets above the zero level (current prices) or into the overbought area (20-30 pips above the current prices).
Read more on how to apply Fibonacci studies to calculate price targets.