What was St. Ogg's profession in The Mill on the Floss?

Let us be careful here. Your question seems to suggest
that St. Ogg's is actually a character in this excellent novel. However, it is actually
the town in which the novel is set, as the opening chapter makes clear. Eliot seems to
like beginning her novels with an introductory passage that clearly establishes setting
before introducing us to her main characters, and this novel is no exception as we are
presented with the beautiful and tranquil image of St. Ogg's and the surrounding
countryside.

If your question was refering to the industry
of St. Ogg's, the description makes it clear that this town owes much of its prosperity
to the way in which it is clearly an important harbour
town:

On this
mighty tide the black ships--laden with the fresh-scented fir-planks, with rounded sacks
of oil-bearing seed, or with the dark glitter of coal--are borne along to the town of
St. Ogg's, which shows its aged, fluted red roofs and teh broad gables of its wharves
between teh low wooded hill and the river brink, tingling the water with a soft purple
hue under the transient glance of this February
sun.

Thus we can see that in
addition to agriculture, St. Ogg's is clearly a prosperous town because of its harbour
and wharves, and the way that important products are shipped there to be
sold.