Hi Kimber, and welcome to the forum.
The village of O'Cebreiro itself is on a mountaintop. I saw people arriving there by taxi, and I think you can get a bus to somewhere just a few kilometres away. Others will be able to provide better transport details.
But there was one point I wanted to mention, re accommodation in O'Cebreiro if you haven't walked there. When I was there late last June, the pilgrim albergue was full, and they were strictly applying the 'priority' rules, that give first priority to walkers, followed by cyclists. The cyclists were having to wait until evening to see if there were any beds left for them. A Spanish family group, with two women and four teens, arrived by taxi early in the evening. As they had not walked, they had no priority for a bed in the albergue and they ended up sleeping up outside in the albergue porch. They had not expected this, and had a very cold night of it. (Other accommodation options in the village seem to be quite a bit more expensive, though you can book some of them ahead of time I think.)
All the best for your planning. I had gorgeous views the day I was on O'Cebreiro, and I hope you have the same good fortune!
2002 CF: 2004 from Paris: 2006 VF: 2007 CF: 2009 Aragones, Ingles, Finisterre: 2011 X 2 on CF: 2013 'Caracoles': 2014 CF and Ingles 'Caracoles":2015 Logrono-Burgos (Hospitalero San Anton): 2016 La Douay to Aosta/San Gimignano to Rome:
There's a bus from Madrid (http://www.alsa.es) that departs at 00:30 and arrives at 05:55. So, you can begin walking at this time, arriving to take a good breakfast in O Cebreiro before beginning you "normal" walk to Triacastela.
It would be a hard day, sleeping not very well and having to walk 5 km. else but nice, very nice.