Invalid Expression Error

I am trying to write a script that runs a select by location then a subset selection of select by attribute but I keep getting an invalid expression error. The last where clause is to selected any blank fields in the STD_CND field.

arcpy.SelectLayerByLocation_management("Culvert_subset", "INTERSECT", "Cont_Buff")
arcpy.SelectLayerByAttribute_management("Culvert_subset","SUBSET_SELECTION",'"STD_CND" = poor' or '"STD_CND = Very Poor' or '"STD_CND" = ""')

Additionally once this selection is made will it continue (or be saved) through the rest of the script or should I create a new shapefile from it?

Any thoughts?
Thanks!