It is possible that some may think that since Oliver in Oliver Twist is a highly representative character who doesn't really undergo an epiphany or change of heart or adoption of a new way to be, then he is an abstract character. Since Oliver doesn't undergo major character development as denoted above, he is described as a flat character. Flat characters are commonly used in adventure stories because the point of an adventure story is to experience, resolve, and survive the adventure. It is possible to see Oliver Twist as an adventure story because the adventures (or misadventures) Oliver experiences must certainly be gotten through and we certainly do hope they are resolved and that he does survive. In which case a flat character is a suitable one for Oliver. It is also possible to give an allegorical reading to Oliver Twist especially since Dickens stated that his intent was for Oliver to represent goodness that survives and triumphs at last.
Based on this, if the opening supposition of a representative character being an abstract character is accepted, then the way to make Oliver a concrete, non-representative character, is to give him significant character development. He can't be given a significant weakness and still fulfill Dickens' intent but his extreme naivete could reasonably render him less astute than Dickens paints him. In this way, his character development could relate to developing a vivid understanding of the good and evil sides of life. In such a scenario, for example, perhaps he would need guidance before he could exclaim that God should forgive Fagin.