Prompted by a perceived lack in ship numbers when compared with the British Royal Navy, the O class' design was born with the suggestion of modifying the P-class cruiser design with 380 mm (15 in) guns instead of 283 mm (11.1 in).

The O class' design reflected their intended role; a heavy main armament (six 380 mm guns in three dual turrets) for possible encounters with escorting 203 mm (8 in)-armed heavy cruisers, enough armor to defend against the same and nothing more, and a high top speed so that they could get away from slower but much better armored capital ships.

The required maximum speed of 34 knots (63 km/h; 39 mph) meant that the minimum length had to grow from the original 217 to 229.5 m (712 to 753 ft).

Unfortunately for the designers, the widened beam meant that an even longer hull was needed to maintain hydrodynamic efficiency.

[b] The most persuasive argument for increasing the armament came in 1939, when Adolf Hitler denounced the 1935 Anglo-German Naval Agreement.

[3] Although interest in the P-class ships and the battlecruiser proposal waned for a time in late 1937 and early 1938, it was quickly revived on 28 April 1938 with Hitler's public airing of his views on the 1935 Anglo-German Treaty and the consequent increased possibility of war with the United Kingdom.

Calling the Naval Staff and Admiral Erich Raeder to him, Hitler laid out his thoughts of a strong navy that could pose enough danger to the British such that they would enter into an alliance with Germany rather than go to war.

As decided here, Plan Z entailed two task forces each centered on three H-class battleships and one aircraft carrier, with cruisers and destroyers as escorts.

In war, these forces would collaborate with the three battlecruisers by occupying convoy screens while U-boats and one or more of the O class destroyed the cargo-carrying merchant ships.

It was enough that the design team decided that it would not be prudent to use an all-diesel arrangement; instead, a hybrid diesel-steam turbine propulsion plant was to be used.

In addition, the dockyard personnel necessary for the ships' construction were by now occupied with more important work, primarily on new U-boats.

The ships were to be of welded steel construction, with twenty watertight compartments and a double bottom that extended for 78% of the length of the hull.

The ships were also to be equipped with a double catapult mounted between the two funnels, and four Arado Ar 196 seaplanes for maritime reconnaissance.

Eight MAN 24-cylinder V-configuration two-stroke diesel engines powered two Vulcan gearboxes, both of which drove the outer pair of 4.85 m (15.9 ft) diameter three-bladed screws.

These supplied steam for one set of Brown, Boveri & Cie turbines, which provided power for the center 4.9 m (16 ft) three-bladed screw.

Eight 920 kW diesel generators supplied electrical power for the ships, for a total of 7360 kW, at 220 V.[6] The ships' main armament batteries were to have consisted of six 38.1 cm (15 in) L/47 SK C/34 quick-firing guns mounted in three twin turrets, the same Drh LC/38 gun turrets as were used on the Bismarck-class battleships.

[10] The secondary battery consisted of six 15 cm (5.9 in) L/48 quick-firing guns, also mounted in three twin turrets; two were placed on either side of the forward superstructure, the third was placed aft of the main superstructure, superfiring over the rear main battery turret.

The main armored belt was 190 mm (7.5 in) thick in the most critical areas of the ship, including the machinery spaces and ammunition magazines, and 100 mm (3.9 in) in other less important areas; the armor tapered to zero at the stern and bow of the ship.

Critical areas were further protected by 80 mm-thick shields set back from the torpedo bulkhead.