He was also a member of the Reichstag from 1907 to 1911, arguing for tax breaks for retail traders, the public control of corporate cartels and syndicates and for changes to the Prussian election law.

As the war casualties mounted, Oeser urged families to have many children and he also favoured giving women the same political and social status as men.

[1] After the end of the war, Oeser was once again a member of the Prussian diet, first of the Landesversammlung, the constituent assembly of Prussia in 1919-21 and then 1921-24 of the Landtag, this time for the German Democratic Party (DDP).

The next day, Oeser and Prussian Minister of Finance Albert Südekum [de] demanded Kapp's resignation and threatened him with a strike by railway workers.

He supported the policy of passive resistance, despite the damaging effect it had on the German economy, thinking it might be used not just to end the Ruhrkampf but also to achieve a revision of the much-despised Treaty of Versailles.

In the coalition crisis of November 1923 he favoured an exit by the Social Democrats from the cabinet but overestimated their willingness to tolerate a minority government.

In the subsequent cabinet of Wilhelm Marx Oeser prepared the Reichsbahn for its independence as a formally private institution.