The eight I Squadron ships went into the Baltic on 22 February 1915 for unit training, which lasted until 13 March. Following their return to the North Sea, the ships participated in a series of uneventful fleet sorties on 29–30 March, 17–18 April, 21–22 April, 17–18 May, and 29–30 May. "Ostfriesland" and the rest of the fleet remained in port until 4 August, when I Squadron returned to the Baltic for another round of training maneuvers. That month, "KzS" Ernst-Oldwig von Natzmer replaced Engelhardt as the ship's commanding officer. From the Baltic, the squadron was attached to the naval force that attempted to sweep the Gulf of Riga of Russian naval forces in August 1915. The assault force included the eight I Squadron battleships, the battlecruisers , , and "Seydlitz", several light cruisers, 32 destroyers and 13 minesweepers. The plan called for channels in Russian minefields to be swept so that the Russian naval presence, which included the pre-dreadnought battleship , could be eliminated. The Germans would then lay minefields of their own to prevent Russian ships from returning to the Gulf. "Ostfriesland" and the majority of the other big ships of the High Seas Fleet remained outside the Gulf for the entirety of the operation. The dreadnoughts and were detached on 16 August to escort the minesweepers and to destroy "Slava", though they failed to sink the old battleship. After three days, the Russian minefields had been cleared, and the flotilla entered the Gulf on 19 August, but reports of Allied submarines in the area prompted a German withdrawal from the Gulf the following day. By 26 August, I Squadron had returned to Wilhelmshaven.