In this capacity, he also joined the parliament's Council of Elders, which – among other duties – determines daily legislative agenda items and assigning committee chairpersons based on party representation.

[16] In the negotiations to form a so-called Grand Coalition following the 2013 federal elections, Oppermann led the SPD delegation in the internal and legal affairs working group; his co-chair was Hans-Peter Friedrich of the CSU.

When Frank-Walter Steinmeier resigned as Chairman of the SPD Parliamentary Group to serve once again as foreign minister in Angela Merkel's second Grand Coalition, Oppermann was elected as his successor on 16 December 2013.

[18] In late 2015, the SPD's board under the leadership of Sigmar Gabriel mandated Oppermann and Manuela Schwesig with the task of drafting an electoral program for the 2017 federal elections.

[23] In 2011, Oppermann publicly spoke out in favor of holding a national referendum over fundamental principles of the European Union on the day of the 2013 German federal election.

[24] In 2013, Oppermann criticized the government of Chancellor Angela Merkel as news emerged of its intentions to sell arms to Saudi Arabia, saying that the conservatives wanted to "totally upgrade" the country's military capabilities.

[25] Faced with 800,000 migrants arriving in Germany in 2015, Oppermann said his party would never accept a "CSU proposal to create 'transit zones' near the border, where asylum seekers with no chance of staying could be quickly sent back home".

[26] When a former member of parliament Sebastian Edathy in December 2014 appeared before a Bundestag inquiry into his purchase of child pornography, he was asked about whether a tip-off from party colleagues gave him time to destroy evidence ahead of a police raid on his home and office.

[40] During a closed-door hearing of the Committee on Internal Affairs earlier that year, Oppermann had denied that he or any of his fellow high-ranking SPD officials "indirectly or directly informed or even warned Sebastian Edathy of the investigation or our knowledge of it.