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When Oswy dies he is succeeded by Ecgfrith(as per OTL). Now let us suppose that he is more successful militarily than in OTL.
So in 674 he defeats Wulhere of Mercia and maintains Northumbrian supremacy over Mercia. Then in the Battle of the Trent in 679 he defeats Aethelred (Wulfhere's brother) and Lindsey (Northern Lincolnshire) is fully absorbed into Northumbria proper. Finally in 685 he defeats the Picts (not necessarily at Dun Nechtain/Nechtainsmere where he was killed OTL).
Now this will create a stronger more stable Northumbria but would it be enough to see off the Vikings 100 or so years later? Or at least redirect them south as there will no longer be any easy pickings North of the Wash on the east coast of England?