Berrade, 26, gave his Kern Pharma team their third stage win of this year's race, powering over the line in his home region four seconds ahead of Swiss Mauro Schmid. Another Kern Pharma rider Pau Miquel crossed the line in third.
O'Connor, riding for the Decathlon-AG2R La Mondiale team, held his five-second advantage on Slovenian three-time champion Primoz Roglic after the medium mountain stage over 179.5km from Vitoria-Gasteiz to Maeztu.
Friday's 19th stage arrives at the summit of Alto De Moncalvillo in La Rioja region, with another decisive mountain top finish on Saturday before the final time trial in Madrid on Sunday.
