Bring water and ¼ tsp. salt to a boil. Add potatoes and cook until tender. Drain, reserving 1 cup of cooking liquid (add water to measure 1 cup if necessary). Melt butter in a medium saucepan over medium heat. Sauté chopped scallions in the hot butter until tender. Add flour to butter and onion mixture; stir until smooth. Season with a little pepper. Add in oregano and celery salt. Continue to cook and stir for 1 minute. Gradually add potatoes, reserved 1 cup of cooking liquid and milk. Stir well. Add cheese and ham. Simmer potato ham soup over low heat, stirring frequently, for about 20 minutes.