I have noticed many census references that give Onchan as the Town/Parish followed by a Douglas Address.

The confusion arises because the majority of the town of Douglas is physically in the Parish of Onchan (though for eccliastical purposes it is in Braddan).

Although the town of Douglas was enumerated separately from the parishes of Onchan and Braddan, on-line databases such as Ancestry.com and Familysearch.org continue to list the town under the parish names.

This is probably significant only if you are trying to find an address on a map.