Oloron Ste Marie is on the Camino d'Arles. I walked from Oloron Ste Marie through the Somport Pass, where the Camino Aragones begins, to just before Puenta la Reina, where it joins the Camino Frances from St Jean pied de Port, and on to Santiago on the Frances, in 2016. To get to Oloron Ste Marie, I flew to Paris, then on to Pau, bus into town and a train to Oloron Ste Marie. I purchase a guidebook on the route to Puenta la Reina from the Confraternity of St James in London. That was part ii. Toulouse to Puente la Reina of the Arles to Puente la Reina guide, with the 2015 update insert. I walked in the fall, from September to November. This guidebook is available on their website, but will not be sent out until they are able to open up after the pandemic. The guidebook will probably contain a printed insert update from recent pilgrims, so do not be concerned if the date of publication is some years ago. There are many options for guidebooks onward from Puenta la Reina. And of course, much is likely to change this year, so more recent guidebooks on the Frances will be available when you are ready to go. I enjoyed the autumn walk, which that year had only a couple of rainy days. Enjoy your planning. There is much up-to-date information on this forum from pilgrims who walked the route more recently, search under camino aragones. Enjoy your planning, for a truly remarkable walk.
Edited for weather: From the Somport Pass, the weather was largely sunny, but the three day walk from Oloron Ste Marie up to the pass was very wet, as is common in the Vallee d'Aspe. In fact, a storm had led to a complete power outage when I arrived in Oloron Ste Marie, and I wandered the town in my rain gear, thoroughly soaked and looking for someplace to get something to eat. Maybe I wanted to forget about this, but the valley weather is apparently often wet.