Oracle Index Rebuild and Table Write

A big table ( 10 GB ) which has an index on a number field. Index is almost same size as the table and I want to rebuild the index.The database is very active and the table is also very active read about 100K times in 20 minutes and few writes as well.Index rebuild may be needed when  there are a significant number of deletes, and also do significant index range scans . When index rebuild process, does oracle allow writing to the table ( insert into that table) ?

We can use the online keyword then DML can happen concurrently,if not, then not.

ALTER INDEX ... COALESCE instead of REBUILD.  It is transactional and can be executed concurrently with DML with much less risk (and less licensing) than ALTER INDEX ... REBUILD ... ONLINE