O'Cebreiro

It could depend on the hospitalero - one might say OK and another say "No!"
I don't think it's right to turn a pilgrim away when it is their first day. Thousands catch a bus to Roncesvalles and stay in the albergue there with no problem.
What time do you get there?
If late, the albergue could be full. In which case you could book a place ahead of time.
If early, you could walk on to another albergue.

The night I stayed in the albergue in O'Cebreiro, it did get full, and they rigidly applied 'priority' there. The cyclists who wanted to stay had to wait until quite late -maybe it was even after 8pm- to see if there was room. A couple of groups, including teenagers and adults, arrived by taxi to begin in O'Cebreiro. They were lowest in priority, and one of these family groups ended up sleeping outside the albergue, on what was a very chilly evening on a mountaintop. However, if you have walked 5km, you have walked at least something, so as sil says, it might then depend on the hospitalero.
Margaret

Possibly it will not be a problem.

But, the other option is to continue to Triacastela (only 21 km).

Buen Camino,

Javier Martin
Madrid, Spain