By default, Omni Loader will create all of the tables in the target databases, dropping them beforehand if they already exist. If you already have target tables in place, you can opt to load data only and not touch the target schema. To do so, select Expect existing instead of Create.
By default, Omni Loader will copy all of the data. To skip data copying (for example, if you only wish to create the target schema), select None. To copy newly inserted records, select Delta mode, which copies all records with higher primary key value in the source than the current maximum in the target table.
You may want to turn this off if you expect tables to already exist.