Active transactions with completed statements that refer to data in that tablespace are not affected at the transaction level. When the tablespace is brought back online, Oracle applies the rollback data to the tablespace, if needed. If a tablespace was offline when you shut down a database, the tablespace remains offline when the database is subsequently mounted and reopened. An offline tablespace cannot be read or edited by any utility other than Oracle. Thus, tablespaces cannot be transferred from database to database.
Users trying to access tables in the offline tablespace receive an error.
However, special circumstances can occur when tablespaces are taken offline. If the tablespace containing the tables is offline, the table data in the database is not accessible because the tables are required to access the data. In summary, if Oracle has enough information in the online tablespaces to execute a statement, it will do so. If it needs data in an offline tablespace, then it causes the statement to fail. The primary purpose of read-only tablespaces is to eliminate the need to perform backup and recovery of large, static portions of a database.