diff --git a/core-spec/osi-schema.json b/core-spec/osi-schema.json
index 72cb164..4258fd0 100644
--- a/core-spec/osi-schema.json
+++ b/core-spec/osi-schema.json
@@ -108,6 +108,54 @@
       "required": ["dialects"],
       "additionalProperties": false
     },
+    "Source": {
+      "description": "Dataset source. Legacy string sources are supported for backward compatibility.",
+      "oneOf": [
+        {
+          "type": "string",
+          "description": "Legacy source string for a physical table/view reference or query.",
+          "deprecated": true
+        },
+        {
+          "$ref": "#/$defs/TableSource"
+        },
+        {
+          "$ref": "#/$defs/QuerySource"
+        }
+      ]
+    },
+    "TableSource": {
+      "type": "object",
+      "description": "Object-backed dataset source.",
+      "properties": {
+        "kind": {
+          "const": "table",
+          "description": "Identifies this source as an object-backed dataset."
+        },
+        "object": {
+          "type": "string",
+          "description": "Reference to the underlying physical table/view, e.g. database.schema.table."
+        }
+      },
+      "required": ["kind", "object"],
+      "additionalProperties": false
+    },
+    "QuerySource": {
+      "type": "object",
+      "description": "Query-backed dataset source.",
+      "properties": {
+        "kind": {
+          "const": "query",
+          "description": "Identifies this source as a query-backed dataset."
+        },
+        "object": {
+          "$ref": "#/$defs/Expression",
+          "description": "Query expression with dialect support."
+        }
+      },
+      "required": ["kind", "object"],
+      "additionalProperties": false
+    },
     "Dimension": {
       "type": "object",
       "description": "Dimension metadata",
@@ -163,8 +211,7 @@
           "description": "Unique identifier for the dataset"
         },
         "source": {
-          "type": "string",
-          "description": "Reference to underlying physical table/view (database.schema.table) or query"
+          "$ref": "#/$defs/Source"
         },
         "primary_key": {
           "type": "array",
diff --git a/core-spec/spec.md b/core-spec/spec.md
index ea7e775..f87dbd2 100644
--- a/core-spec/spec.md
+++ b/core-spec/spec.md
@@ -82,7 +82,7 @@ Logical datasets represent business entities or concepts (fact and dimension tab
 | Field | Type | Required | Description |
 |-------|------|----------|-------------|
 | `name` | string | Yes | Unique identifier for the dataset |
-| `source` | string | Yes | Reference to underlying physical table/view (e.g., `database.schema.table`) or query |
+| `source` | string/object | Yes | Structured source definition. Legacy string sources are still supported for backward compatibility. |
 | `primary_key` | array | No | Primary key columns that uniquely identify rows (single or composite) |
 | `unique_keys` | array of arrays | No | Array of unique key definitions (each can be single or composite) |
 | `description` | string | No | Human-readable description |
@@ -109,12 +109,37 @@ unique_keys:
   - [first_name, last_name]    # Composite unique key
 ```
 
+### Source Examples
+
+```yaml
+# Legacy string source, deprecated but still supported during migration
+source: sales.public.orders
+
+# Object-backed source
+source:
+  kind: table
+  object: sales.public.orders
+
+# Query-backed source
+source:
+  kind: query
+  object:
+    dialects:
+      - dialect: ANSI_SQL
+        expression: |
+          select order_id, customer_id, revenue
+          from sales.public.orders
+          where revenue > 10000
+```
+
 ### Example
 
 ```yaml
 datasets:
   - name: orders
-    source: sales.public.orders
+    source:
+      kind: table
+      object: sales.public.orders
     primary_key: [order_id]
     unique_keys:
       - [order_id]
@@ -430,7 +455,9 @@ semantic_model:
 
     datasets:
       - name: orders
-        source: sales.public.orders
+        source:
+          kind: table
+          object: sales.public.orders
         primary_key: [order_id]
         description: Customer orders
         fields:
@@ -465,7 +492,9 @@ semantic_model:
             description: Order amount
 
       - name: customers
-        source: sales.public.customers
+        source:
+          kind: table
+          object: sales.public.customers
         primary_key: [id]
         description: Customer information
         fields:
diff --git a/core-spec/spec.yaml b/core-spec/spec.yaml
index ed46203..f4d5c91 100644
--- a/core-spec/spec.yaml
+++ b/core-spec/spec.yaml
@@ -66,8 +66,18 @@ datasets:
   - name: string
 
     # Required: Reference to the underlying physical table/view or query
-    # Format should be either database_name.schema_name.table_name or query
-    source: string
+    # Legacy string source remains supported for backward compatibility.
+    source:
+      kind: table
+      object: string
+
+    # Query-backed source:
+    # source:
+    #   kind: query
+    #   object:
+    #     dialects:
+    #       - dialect: string
+    #         expression: string
     
     # Optional: Primary key definition that uniquely identifies rows in this dataset
     # Can be a single column or a composite of multiple columns
diff --git a/python/src/osi/__init__.py b/python/src/osi/__init__.py
index 00970fd..f2aba75 100644
--- a/python/src/osi/__init__.py
+++ b/python/src/osi/__init__.py
@@ -10,8 +10,11 @@
     OSIExpression,
     OSIField,
     OSIMetric,
+    OSIQuerySource,
     OSIRelationship,
     OSISemanticModel,
+    OSISource,
+    OSITableSource,
     OSIVendor,
 )
 
@@ -27,7 +30,10 @@
     "OSIExpression",
     "OSIField",
     "OSIMetric",
+    "OSIQuerySource",
     "OSIRelationship",
     "OSISemanticModel",
+    "OSISource",
+    "OSITableSource",
     "OSIVendor",
 ]
diff --git a/python/src/osi/models.py b/python/src/osi/models.py
index 766b6e0..442f6bd 100644
--- a/python/src/osi/models.py
+++ b/python/src/osi/models.py
@@ -1,5 +1,5 @@
 from enum import Enum
-from typing import Any, Optional, Union
+from typing import Any, Literal, Optional, Union
 
 import yaml
 from pydantic import BaseModel, ConfigDict, Field
@@ -66,6 +66,27 @@ class OSIExpression(BaseModel):
     dialects: list[OSIDialectExpression]
 
 
+class OSITableSource(BaseModel):
+    """Object-backed dataset source."""
+
+    model_config = ConfigDict(frozen=True)
+
+    kind: Literal["table"]
+    object: str
+
+
+class OSIQuerySource(BaseModel):
+    """Query-backed dataset source."""
+
+    model_config = ConfigDict(frozen=True)
+
+    kind: Literal["query"]
+    object: OSIExpression
+
+
+OSISource = Union[str, OSITableSource, OSIQuerySource]
+
+
 class OSIDimension(BaseModel):
     """Dimension metadata on a field."""
 
@@ -94,7 +115,7 @@ class OSIDataset(BaseModel):
     model_config = ConfigDict(frozen=True)
 
     name: str
-    source: str
+    source: OSISource
     primary_key: Optional[list[str]] = None
     unique_keys: Optional[list[list[str]]] = None
     description: Optional[str] = None