Most Oneworld carriers have round-trip flights to various destinations in Europe at a discount today for $418-643, mostly with a connection. Dates vary, use the linked calendar. Cheapest fares are mostly midweek. Most fares are not reversible.
This recent sale started due to Oneworld dropping their prices and the other airlines matching.
Most destinations have flights from January to May and also September to November. The exact ranges for each destination will vary. Use the Google calendar view to get an idea of what dates work.
These type of regional sales often get matched by the other major airline alliances within a day or so. If you decide to purchase, then you should continue to monitor your price afterwards, it may drop lower within the 24-hour free cancellation window allowing you to re-book a cheaper flight.
Flights to smaller airports may require a long connection or two stops. When you begin your search, modify your filter to remove fares that have more than one connection, some of those smaller airports ONLY have flights with bad routing.
