Help me to understand VOOC fast charging for lithium ion batteries?
Li-ion cell charging voltage ranges from ~3.7V to 4.2V. 18W/4.2V = 4.29A. The charger has a DC/DC voltage converter circuit which pulses current through an inductor to 'transform' the voltage down and current up. This process is typically 85~90% efficient, so it should be able to deliver 4A for most of the charge cycle.


  But again how can just 5V can charge battery to 4A rating?


20W/4.2V = 4.76A, or ~4A at 85% efficiency. The OPPO VOOC (Voltage Open Loop Multi-step Constant-Current Charging) 'Flash Charging' system claims to charge certain OPPO devices from 0 to 75% in 30 minutes, implying a capacity of ~2.7Ah. 

At 4A a 4Ah cell would only get 50% charge in 30 minutes. Charging to 75% would take (4Ah x 75%) / 4A = 45 minutes.

At 75% charge the cell has probably reached 4.2V, then current must be reduced to avoid damaging the battery. Getting The last 25% into the cell may take another 20-30 minutes.