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The concept of "open jaw" does not apply to a one-way.
You can use your Avios for travel on Niki with no surcharges, and for travel on BA in or out of LON with a low co-pay if you qualify for a "Reward Flight Saver" redemption.
It's possible that Icelandic offers a free or low-cost stopover at KEF on a one-way cash ticket from Europe to NYC.
AA's one-way international awards used to offer a free North American "gateway" stopover. Unfortunately, AA did away with the gateway stopover option in April of this year.