Recently, the trade compensation needed to get Fields was predicted by ESPN's Jeremy Fowler, and he stated that the going rate for the young quarterback would be a second-or-third round pick. While just giving away a second-rounder without anything else would not make sense, adding Fields for the No. 73 overall pick could be a solid deal for the Raiders.
Round 3 picks are usually hit-or-miss at the NFL level, and as we saw last year, with the Raiders picking Alabama defensive tackle Byron Young there, misses often happen. By trading away the No. 73 overall selection, the Raiders would be giving away a top-100 selection, but in return, they would get getting a playmaker at the quarterback position.
There were many times this season where O'Connell could have made a play with his legs, but his lack of mobility hurts him in that department. That would not be a problem for Fields, who can dominate the game on the ground, or through the air, and would have the best weapons he has ever had at his disposal in Las Vegas.
You also have to look at how bad the Bears offensive line has been at times during Fields's tenure as quarterback, and that has not helped him at all in his development. The Raiders offensive line was much better than advertised, and with many of the starters coming back, Fields would have the best protection he has had in his career as well.