I haven't been to Ourense, or to Lugo for that matter. I'd rank Santiago de Compostela as the top destination in Galicia, but I also liked A Coruna and Pontevedra. Vigo has a tiny historic area near the port, but I thought is was imminently skippable, and it's probably the only place I went during my 3-month trip to Spain about which I'd say that. Not a bad place, just not very distinctive.
I also enjoyed several hours wandering around Betanzos, which is quite near A Coruna.
There are some attractive smaller towns, too, but January is probably not the best time of year for small coastal places that have shifted from fishing villages to tourist destinations. I'm thinking of Cambados and Combarro in particular. Tui isn't coastal, but it is quite small and awkward to get to; I think I'd go elsewhere in mid-winter.
If you care about weather, you might check wunderground.com for info on what is to be expected in January. In the summer, A Coruna seemed notably warmer and sunnier than much of the rest of Galicia, and Santiago de Compostela is notorious for getting little sun.