It looked like Rickon(the kid) and they killed Osha off. I'm not sure it's a ruse, or if it is one, it's definitely not doing Osha/Rickon any favors.
Well, 2 things.
1.) Yeah, it might not be for Rickon, or the Starks--this is a personal vendetta for the death of his father. Rickon is a piece being played so he can lure Ramsay into trusting him and get him out of the castle.
2.) Rickon might be willing to take the risk. Remember, people constantly underestimate how depraved Ramsay is--Rickon might believe he'll be treated like any other high level capture, and kept alive to negotiating purposes--so he agrees to play in the Ruse.
This might be taking the place of White Harbor and House Manderly in the books. Where Great Jon's son is pissed off his father was betrayed and killed by the Boltons--so he, himself, is plotting revenge (Not FOR the Starks, but perhaps with Rickon). The revenge isn't buried in clever exposition though, its just hidden from the audience directly. The goal is to get Ramsay to commit to taking action against the Wildlings and thus get Ramsay OUT of Winterfell and in a position where Umber forces can throw the Red Wedding 2.0. (Because remember, they went over how well provisioned Winterfell is, during the winter--there is no way to siege them.)
It wouldn't make any sense at all to be a ruse. Unless the Umbers have been having double secret talks with Jon Snow that the show somehow forgot to mention, they have every reason in the world to want to ally with Ramsay and kill the wildling army.
However things end up going down in the Jon v Ramsay conflict I don't see a way out of this for Rickon. Maybe some secret tunnel escape but is that even likely for a character who's pretty much a non starter? Snow is the main man in the story and the north.
I don't know why'd they'd need Jon for this to be a ruse. The goal of the ruse is for Umber and Rickons revenge, not to put Snow on the throne. Winterfell is unassailable with a siege, especially in winter. But if Ramsay were to leave the castle--say to help the Umbers with a wildling threat? Then he'd be vulnerable to a double cross from the Umbers .
Jon Snow doing his big march may have actually fucked up their plans, they were probably depending on him doing nothing and leaving the wildlings as bait for the plan (If this was a double cross).
But long and short it's not like the Umber's don't have scourts--if they saw the Wildling army was just a bunch of women and children, they could get the best of both worlds. They could tell Ramsay that constant fighting has drained their forces. Then get Ramsay up there with his army, and only commit half their army, while hiding the other half. Have him attack the Wildlings, in the chaos have the hidden contingent of Umbers swing around and murder Ramsay, finish off the wildlings and then finish off Ramsay's now drained forces. By playing up the fact that he's Ramsay's friend, he gets everything he wants.
And remember--he specifically did not swear an Oath.