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.