Final answer:
o'Brien refers to Syme indirectly to highlight the contrast between beauty and purity and the ugliness and coarseness of the narrator's life.
Step-by-step explanation:
o'Brien refers to Syme indirectly in the passage because there is something about Syme's face and form that the man Wolfe loathes.
The narrator mentions that Syme is different from his fellow workmen and has a finer nature. However, Wolfe feels disgusted by Syme's deformity, even when he is being kind to him.
The mention of Syme serves to highlight the contrast between beauty and purity represented by the little Irish girl Janey and the ugliness and coarseness of the narrator's life in the cellar.