An interface would be the simplest way to accomplish what you are looking for. All you need is the interface definition:

Public Interface IParser
    Function ParserGetContent(ByVal tnPrimaryKey As Integer) As String
End Interface

Then in your base class, add

Implements IParser

after the class definition, and change the type of oOriginator to IParser. Your existing classes would inherit the interface from the base class, and you can add the interface to any new classes that also need that method.