If your plan is to travel from Osnabruck to Munich on Monday in time to join the Night Jet leaving that evening before midnight then that is classed as ONE of your I/O journeys and only needs one travel day.

If though you are travelling Os to Mun on Monday and then catching the Tuesday Night Jet to Rome that will take 2 I/O days and 2 Travel days.

Re the booking for the NJ it seems they are fully booked but others may know different.

What dates?

Thanks for the quick answers!!

Plan was: Monday, the 12th  September from Osnabrück to Munich with the night train 21.37pm and then Tuesday, the 13th September from Munich to Rom with the night train at 20.10pm  - so that means two days of I/B right? And in total you only got two days, so the ride back (Saturday to Sunday) wouldn’t count?

The NJ seems to be completely booked unfortunately… so anyone any idea what to do? Can we still use the NJ if we don’t have any reservations?

Wenn ihr so fahren würdet, sind alle Fahrttage in Deutschland weg. Für die Rückfahrt müsstet ihr dann Fahrkarten kaufen. 

Der Nightjet ist in der Tat ausgebucht. Ohne Reservierung könnt ihr ihn nicht benutzen.  

Der Zugteil nach Mailand hat noch Sitz- und Liegeplätze. 

It seems the train from Os to Mun is simply an overnight ICE, not a sleeper.

If you were to take the 0037 to Munich, change at Cologne, arrive 0833.

Then take the 0934 to Bologna arrive 1619.

Then Bol to Roma - several trains to choose (About 2 hours).

Cheap hotel to recover. 

Low reservation costs (about 20 euro) and only one travel day (plus a lack of sleep)

What @Yorkie has not mension, is that the first ICE to Munich has optional reservations (via DB 4,50€ Köln-Bologna), and you have to pay 10€ fee for the EC Munich-Bologna (online bookable via ÖBB, one way ticket option + as discount Interrail) or you buy this 10€ fee at a DB Reisezentrum and get the reservation for free for the EC Munich-Bologna.

Von Osnabrück nach Köln oder auch eine Station früher wenn die Planmäßige Abfahrtszeit nach Mitternacht ist um damit keinen zusätzlichen Tag zu nutzen, bis dorthin kauft ihr ein normales Ticket.

Thank you all!! Have to admit that it’s more complicated with all these reservations than we thought :D 

@Angelo  your suggestion was:
21.37h Osnabrück to Köln (23.49h Arrival, then leave 23.55h) to München (6.03h Ankunft). From München we take the  EC to  Bologna Arrival 14.10h. -- so that would be one day and would be covered with the Inbound right?

Outbund: We need to be back in Osnabrück at Sunday, the 18th of September. We could leave anywhere  from Italy but I cannot find any NJ which are still free - is there an overview so that we can see which trains from ÖBB still have free reservations?

There is one key difference in leaving on the 21,37 to Cologne compared to the 0037. The 21.37 routing takes 2 travel days, whereas the 0037 only requires one travel day to get to Rome mid evening.

Reservations are an inevitable feature of travelling on European High Speed Trains and easy to cost in and, for Italy /Austria/Germany reasonably straightforward through OBB,DB or even Interrails own service, which does work reasonably well.

As for your return why are you so keen on a sleeper? They are expensive if you want a cabin and couchettes are basic and shared. A good B&B in Bologna is about 80 euro a night for 2 people, hostels even cheaper.

Something like a direct train from Venice to Vienna (daytime) and then a cheap hotel before using your inward journey next day, or again Bologna to Munich by 1830 and onward the same day to Osnabruck.

For your information, travelling via Switzerland  for your Inbound journey back home you can use Nightjet/IC service Zürich-Basel-Hamburg, I have no idea if it runs via Osnabrück-Bremen or via Hannover but you could pay a Ticket for the leg the day after to get in the case from Hannover to Osnabrück.

But if I have the pass only for Italy would that still count as an inbound/outbund if I go via Switzerland and then back to Germany?

A one country pass for Italy is only valid in Italy and there are no included inbound/outbound journeys.