Social Darwinism refers to the way in which Darwin's idea of Survival of the Fittest was exploited to justify a series of approaches that resulted in social policies that did not differentiate between those who could support themselves and those who couldn't, because, arguably, the fittest of individuals would survive and the unfit ones would die, which would be a good thing for human race. Of course, so much of the writings of Dickens argued against such an approach and tried to present the conditions and context of the poor which of course set them at such a disadvantage.
Of course, the way that Oliver Twist is brought up and the labour he has to do is a perfect example of the picture of poverty that Dickens was trying to expose to his middle and upper class readership. In particular, one character who expresses his views about Oliver's fitness for life is the famous gentleman in the white waistcoat, who again and again refers to the fact that he believes Oliver will hang because of his unfitness for life and the way in which he has the temerity to ask for more. He goes a step further after Oliver manages to escape being apprenticed to a chinmey sweep:
That same evening, the gentleman in the white waistcoat most positively and decidedly affirmed, not only that Oliver would be hung, but that he would be drawn and quartered into the bargain.
Characters such as this gentleman and Mr. Bumble, by referring to their generosity in providing for Oliver and giving him a good start in life, whilst ignoring the very real social barriers and issues that Oliver faces, can be used to express Social Darwinism.