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I would not characterize the datafiles as being "corrupt" when Oracle crashes. A better term is that the datafiles will be "inconsistent" after an instance crash. The files will be inconsistent because there may be uncommitted transactions in the datafile. So the online redo log files are used to resolve the uncommitted data. It is also possible that the datafiles will not contain the results of a committed transaction.
It is highly possible that when the instance crashes, there are uncommitted transactions in the redo log files as well. This means the online redo log files share the same inconsistency. However, when you start the instance, SMON performs instance recovery which means the online redo log files are used to commit any changes not yet written to the database data files, as well as rollback any uncommitted transactions that were in progress during the crash.
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