Today I experimented this nice oracle error at startup:

ORA-19815: WARNING: db_recovery_file_dest_size of 4G bytes is 99.76% used, and has 9895936 remaining bytes available.

You can check your RECOVERY size with

SELECT name, 
   space_limit/(1024*1024) limit in MB, 
   space_used/(1024*1024) used in MB, 
   space_reclaimable/(1024*1024) reclaimable in MB
FROM
   V$RECOVERY_FILE_DEST;

*In my case* the fastest way to solve was to mount readonly and grow the DB_RECOVERY_FILE_DEST_SIZE:

#sqlplus / as SYSDBA;
sqlplus# startup nomount;
sqlplus# ALTER SYSTEM SET DB_RECOVERY_FILE_DEST_SIZE = 10g SCOPE=BOTH SID='*';

There are plenty of other use cases, please check what does fit for you!