Yes this would count as 2 inbound/outbound days. You generally have 2 days but maybe you have 3 (trial from Interrail). Activate your pass for a date in September, October,... and see what you get.

The easy solution would be to buy a ticket to Hannover or to use your Deutschlandticket if you have it.

Yes, that are 2 travel days within Germany. But the train at 20:35 from Oldenburg to Hannover is a regional train (RE1), so you can use a Niedersachsen-ticket: €25 for the first person, €6 for the second person, €5 for the 3th, 4th and 5th person.

Thanks for your replies, a little bit strange because it's one journey with two changes. Nevertheless I think we make it like you mentioned, maybe some of us will have this trial, otherwise Niedersachsenticket.

Thanks

The pass works by days (00:00 - 23:59), not by journeys. On a pass day you can board 1 or 20 trains, it doesn't matter.

You have 2 travel days in your country of residence, not 2 journeys over several days. You need to plan your schedule so that you manage to leave your country only boarding trains during one day and the same for your return.