The easiest route from Oregon to the east side of Nebraska (Missouri River) is the Oregon Trail route used by Oregon-bound immigrants in the 1800s. (They had to find the easiest way for their wagons.) The trail crosses the continental divide at South Pass in Wyoming, the only place wagons could cross the divide, and follows rivers (the Columbia, Snake, Sweetwater, and Platte)for much of it's route. I rode the route in 1990 -- not much climbing and lots of westerly winds.
Cross Iowa north of Des Moines to avoid the hilly southern part of the state. Northern Illinois and Indiana and the western 2/3 of Ohio are very flat. I haven't ridden east of Iowa, but I'd guess sticking close to Lake Erie and then following the Erie Canal would be the flattest route to Albany.