Preheat the oil in a deep-fryer or wok to 180°C/350°F (test by frying a small cube of bread; it should brown in 30 seconds).
In a bowl, mix together the flour, salt and baking powder. Dredge the cod fillets in the flour mixture, shaking off any excess. Reserve the remaining mixture.
Lightly beat the eggs with the milk and olive oil. Mix in the reserved flour mixture and beat until smooth.
Dip the cod fillets into the batter, and deep-fry until crunchy and golden brown. Drain on kitchen paper, and serve straight away