Push Notifications to the QGIS User Interface

You will need to add this line of code to the imports near the top of the file.

from qgis.utils import *

def run(self):
    """Run method that performs all the real work"""
    # Create the dialog with elements (after translation) and keep reference
    # Only create GUI ONCE in callback, so that it will only load when the plugin is started
    if self.first_start == True:
        self.first_start = False
        self.dlg = OsoPluginDialog()
    # show the dialog
    self.dlg.show()
    # Run the dialog event loop
    result = self.dlg.exec_()
    # See if OK was pressed
    if result:
        # Do something useful here - delete the line containing pass and
        # substitute with your code.
        fieldName = self.dlg.mFieldComboBox.currentField()
        iface.messageBar().pushMessage("Field Name", fieldName, level=0)
        pass