Einar is correct, the default value is not available in the ontology module, so at the moment you need to set it in the partial class. 

The whole ontology module will be replaced by a Visual Studio plugin before the summer. The plugin will make it possible to set the default value without writing code.

You can add it manually to the definition file aswell:

example:

        <Property Guid="01c13cf9-0443-47b2-944d-be4b52cd0b1e" CodeName="HideLeftColumn" PropertyClassGuid="68bb6e0a-f394-4311-a6d4-cca8c9534404">
          <DataValues>
            <DataValue Guid="53c3bf6b-a4b4-4f1a-bd32-b5a23ee76fc7" FieldName="hide_left_column" DataValueType="Boolean">
              <Default>false</Default>
            </DataValue>
          </DataValues>
          <Globalization>
            <Lang LCID="1044" Name="Skjul venstre kolonne" />
          </Globalization>
          <LoadByDefault>true</LoadByDefault>
          <Settings />
        </Property>