ODP.NET has recently introduced a new connection string attribute validate connection that should be set to true in order to validate connections coming out of the pool. By default, this property is set to false. This saves having to write any custom code to check for invalid connections coming out of hte pool. The attribute is available beginning with ODP.NET 188.8.131.52.
Note that validation causes a round trip to the database for each connection coming out of the pool, which is very expensive in terms of performance. Therefore, this attribute should be used only if absolutely necessary.