Bend, the one in central Oregon, was originally called Farewell Bend. The name had something to do with the Deschutes River, which flows through Bend. Stories conflict. Some say it had to do with the only fordable portion for miles being there. Others that it was the pullout for rafts and boats before the rapids and waterfalls downstream. The name change is also controversial. Some state that the postal service shortened it to Bend. Other sources say the name was in conflict with Farewell Bend on the Oregon-Idaho border, where the Oregon Trail leaves the Snake River and heads inland. BTW, a presidential candidate was drowned on the Deschutes River at Bend. Frank T. Johns, running on the Socialist Party ticket. During a speech he was making on the campaign trail in 1928, a child fell into the nearby river and he went in to rescue the kid, but wound up drowning.
I don't know where the name North Bend came from. The original name was Yarrow. Coos Bay surrounds North Bend on three sides. It literally bends around the town to the north of it, so perhaps that has something to do with the name. The city of Coos Bay is named after the bay. Apparently they didn't like the original name of Marshfield. In the old days, Jedediah Smith explored the area for the Hudson's Bay Company. If you've seen The Simpons episodes showing Jedediah Springfield, the explorer/founder of Springfield, that's where Matt Groening got the first name from. The last name was from the town of Springfield, Oregon.