The manual says that OANDA already does virtual hedging, implying that in this case I don't need to switch it on at Zorro.

But as the broker charges me for every trade, in my understanding reducing the number of real trades with hedge = 5 would help anyhow.

Can anybody please help?

Experiment with the TradeTest script with an Oanda demo account until you understand the implications. Try different Hedge modes and watch what happens.

As to my knowledge, Oanda does not charge for trades at all. Only for swaps and spreads.