I guess using post hook in config block you can create index. Can you please refer below link and try same for Oracle? Although the given query is for MS SQL server but I guess if you try in similar way it should work

https://discourse.getdbt.com/t/how-to-create-indexes-on-post-hook-for-ms-sql-server-target-db/542

Also you can refer this official DBT link for syntax reference- https://docs.getdbt.com/reference/resource-configs/postgres-configs