Applicable for Oracle 12.1 and onwards.
Basic syntax:
CREATE INDEX MYIDX1 ON MYTABLE(COL1, COL2) COMPRESS ADVANCED LOW;
The above statement will create a compressed index in the connected user's default tablespace, using Oracle Advanced Index Compression.
In my environment, I tried this on a B-tree index on a 62 million row table. The size dropped from 3136 to 2368 MB, a 25 percent reduction.
An index can also be altered to use advanced index compression, like this:
ALTER INDEX MYIDX1 REBUILD COMPRESS ADVANCED LOW;
If it's important to allow DML on the index during rebuild, add ONLINE, like this:
ALTER INDEX MYIDX1 REBUILD COMPRESS ADVANCED LOW ONLINE;