I have seen this issue discussed before, never to my satisfaction. Oedipus is a noble character with the traditional tragic flaw which leads to his downfall. Willy is the more modern tragic hero, described in Miller's essay "Tragedy and the Common Man" (http://theliterarylink.com/miller1.html). His definition of the tragic flaw is interesting: "The flaw, or crack in the character, is really nothing--and need be nothing, but his inherent unwillingness to remain passive in the face of what he conceives to be a challenge to his dignity, his image of his rightful status." Miller argues that the essence of tragedy is still possible in our world because everyone faces challenges to his dignity.
In Willie's case, he struggled in an economic system that "eats the fruit and throws away the peel." Some of it is his own fault ... he places all his hopes and efforts in the wrong places; some of it is the fault of the system that seems people in terms of their economic value.
If I had to summarize, I would say they were similar in the efforts to assume their rightful place in the world. Both lacked knowledge: Oedipus of his origins and actions, Willie of what he needed to be successful in life. Both create the traditional response in this reader: pity for both of them and fear that I could wind up making their mistakes.
Willy Loman ('low man') is not "big enough" to fill the shoes of a tragic hero (who, by definition, is someone admired and even 'bigger than life'). Willy, though affable, is portrayed as a weak and diminished character and has none of the posh and pizzaz of the hero type (such as Oedipus). However, at the end of the story he does sacrifice himself to meet his family's financial needs, even if he does it in a dishonourable way (insurance money by suicide). Whether Willy just chickened out of life's problems or did a 'noble' thing for those he loved is another question to be asked...