Ons is reached by ferry from several mainland ports during the summer season, which runs roughly from June to September; outside those months the island is effectively closed to casual visitors. The main departure points are Sanxenxo, Portonovo, Bueu, Marín, and Pontevedra — crossings take between 30 and 50 minutes depending on the port. Multiple operators run scheduled services; book ahead in July and August, when capacity sells out.
Because Ons is within the Atlantic Islands of Galicia National Park, all visitors must have advance authorisation before arriving — the ferry operators handle this as part of the booking process. Daily visitor numbers are strictly capped, so same-day plans are often impossible in high season. If you want to stay overnight, the island has one officially managed campsite; reservations must be made through the national park website well in advance. There is no hotel or guesthouse. Day-trippers should note the last ferry back: missing it means camping whether you intended to or not.