Oracle provides the undocumented opaque_transform hint to allow distributed databases to communicate.  It's not uncommon to see the opaque_transform hint in a distributed query where Oracle has automatically injected this hint in the remote SQL.

SELECT /*+ OPAQUE_TRANSFORM */ col1, col2 . . . .

I'm told that with an opaque_transform hint added to a subquery, Oracle will transform parts of the SQL into an earlier dialect.  When used in remote distributed queries between versions.  However, we also see queries that do not do transformation with this hint:

SELECT /*+ OPAQUE_TRANSFORM */ count(*)
FROM "AF_FUND_HIERARCHY" "AF_FUND_HIERARCHY_SOURCE_DB"

Transform for syntax conversion?

Others say that opaque_transform is automatically injected into standard remote SQL to convert later SQL constructs into older constructs.  For example, this remote query with a CASE statement might need to have the SQL CASE transformed into a DECODE to communicate with an earlier release of Oracle:

Insert into
   tmp_tst(decode_col,case_col)
SELECT decode(t1.t1_status,'A','S','O','N') dedode_col,
CASE WHEN
   t1.t1_status = 'A'
THEN 'S'
WHEN t1.t1_status = 'O'
THEN 'N' END Case_col
FROM t1;

Rows    Execution Plan
------- ---------------------------------------------------
      0 INSERT STATEMENT MODE: ALL_ROWS
      2 REMOTE OF 'T1' (REMOTE) [ORIGIN9I] SELECT /*+ OPAQUE_TRANSFORM */ "T1_STATUS" FROM "T1" "T1"

Transform for VARRAY to LOB?

Another theory suggests that the opaque_transform is related to the ORA-22997 error, and opaque_transform may be converting opaque data types (abstract data types, (ADT)) into LOB columns for insertion into earlier releases of Oracle:

ORA-22997 - VARRAY | OPAQUE stored as LOB is not specified at the table level

Cause: An attempt was made to specify a VARRAY|OPAQUE column to be stored as LOB at the partition/subpartition/template level. However the VARRAY|OPAQUE column was not specified to be stored as LOB at the table level when the table was created.

Action: Specify the VARRAY | OPAQUE column to be stored as LOB at the table level when the table is created. Alternatively, do not specify the VARRAY | OPAQUE column to be stored as LOB at the partition/subpartition/template level if it is not specified at the table level when the table is created.

Here is an example of the opaque_transform hint from some Oracle Applications SQL:

SELECT /*+ OPAQUE_TRANSFORM */ "INVENTORY_ITEM_ID","ORGANIZATION_ID", "PRIMARY_UOM_QUANTITY","RESERVATION_TYPE","RESERVATION_QUANTITY", "DEMAND_SOURCE_TYPE","DEMAND_SOURCE_HEADER_ID","COMPLETED_QUANTITY", "SUBINVENTORY","DEMAND_CLASS","REQUIREMENT_DATE","DEMAND_SOURCE_LINE", "DEMAND_SOURCE_DELIVERY","DEMAND_SOURCE_NAME","PARENT_DEMAND_ID", "DEMAND_ID","SALES_ORDER_NUMBER","SALESREP_CONTACT","SALESREP_ID", "FORECAST_VISIBLE","DEMAND_VISIBLE","RN1","RN2","RN3","PROJECT_ID", "TASK_ID","PLANNING_GROUP","CUSTOMER_ID","BILL_TO_SITE_ID", "SHIP_TO_SITE_ID","REQUEST_DATE","LIST_PRICE","FIRM_DEMAND_FLAG", "SHIP_SET_ID","ARRIVAL_SET_ID","SHIP_SET_NAME","ARRIVAL_SET_NAME", "END_ITEM_UNIT_NUMBER","ORDERED_ITEM_ID","CTO_FLAG","DEMAND_PRIORITY", "PROMISE_DATE","LINK_TO_LINE_ID","ORIGINAL_INVENTORY_ITEM_ID", "ORIGINAL_SYSTEM_REFERENCE","ORIGINAL_SYSTEM_LINE_REFERENCE", "MFG_LEAD_TIME","AVAILABLE_TO_MRP","ORIGINAL_ORDERED_QUANTITY", "ORIGINAL_COMPLETED_QUANTITY","CUST_PO_NUMBER","CUSTOMER_LINE_NUMBER", "ORDER_DATE_TYPE_CODE","SCHEDULE_ARRIVAL_DATE","LATEST_ACCEPTABLE_DATE", "SHIPPING_METHOD_CODE","ATO_LINE_ID","DELIVERY_LEAD_TIME" FROM "MRP_AN1_SALES_ORDERS_V" "X" WHERE ("ORGANIZATION_ID"=82 OR "ORGANIZATION_ID"=83 OR "ORGANIZATION_ID"=101 OR "ORGANIZATION_ID"=121 OR "ORGANIZATION_ID"=142 OR "ORGANIZATION_ID"=162 OR "ORGANIZATION_ID"=182 OR "ORGANIZATION_ID"=282 OR "ORGANIZATION_ID"=283 OR "ORGANIZATION_ID"=284 OR "ORGANIZATION_ID"=285 OR "ORGANIZATION_ID"=287 OR "ORGANIZATION_ID"=288 OR "ORGANIZATION_ID"=289 OR "ORGANIZATION_ID"=290 OR "ORGANIZATION_ID"=291 OR "ORGANIZATION_ID"=293 OR "ORGANIZATION_ID"=294 OR "ORGANIZATION_ID"=295 OR "ORGANIZATION_ID"=296 OR "ORGANIZATION_ID"=297 OR "ORGANIZATION_ID"=298 OR "ORGANIZATION_ID"=300 OR "ORGANIZATION_ID"=301 OR "ORGANIZATION_ID"=302 OR "ORGANIZATION_ID"=303 OR "ORGANIZATION_ID"=304 OR "ORGANIZATION_ID"=305 OR "ORGANIZATION_ID"=306 OR "ORGANIZATION_ID"=307 OR "ORGANIZATION_ID"=309 OR "ORGANIZATION_ID"=310 OR "ORGANIZATION_ID"=311 OR "ORGANIZATION_ID"=313 OR "ORGANIZATION_ID"=314 OR "ORGANIZATION_ID"=315 OR "ORGANIZATION_ID"=316 OR "ORGANIZATION_ID"=317 OR "ORGANIZATION_ID"=318 OR "ORGANIZATION_ID"=322 OR "ORGANIZATION_ID"=342 OR "ORGANIZATION_ID"=422) AND "ORIGINAL_ORDERED_QUANTITY">="ORIGINAL_COMPLETED_QUANTITY" AND ("RN1">:1 OR "RN2">:2 OR "RN3">:3)