I do believe I have found the answer in Medicare's MLM:
"Observation care is a well-defined set of specific, clinically appropriate services, which include ongoing short term treatment, assessment, and reassessment before a decision can be made regarding whether patients will require further treatment as hospital inpatients or if they are able to be discharged from the hospital. Observation status is commonly assigned to patients who present to the emergency department and who then require a significant period of treatment or monitoring in order to make a decision concerning their admission or discharge.
Observation services are covered only when provided by the order of a physician or another individual authorized by State licensure law and hospital staff bylaws to admit patients to the hospital or to order outpatient tests. In the majority of cases, the decision whether to discharge a patient from the hospital following resolution of the reason for the observation care or to admit the patient as an inpatient can be made in less than 48 hours, usually in less than 24 hours. In only rare and exceptional cases do reasonable and necessary outpatient observation services span more than 48 hours.
When a physician orders that a patient be placed under observation care, the patient’s status is that of an outpatient. The purpose of observation is to determine the need for further treatment or for inpatient admission. Thus, a patient receiving observation services may improve and be released, or be admitted as an inpatient."
What I am gathering is that it will be dependant upon the patient's status (morbidity associated with the procedure), Physcian's orders/ medical judgement of the admitting physician. When the patient enters as outpatient, he is considered outpatient regardless of hour he came in, whether he used a bed or remained in the hospital past midnight...unless determined otherwise, ie., a valid order to admit as inpatient.