Final answer:
The most important action when treating a patient suspected of having a stroke is to assess their vitals and neurological status and establish the timeline of symptoms onset.
Step-by-step explanation:
A 58-year-old male presenting with confusion, right-sided weakness, and slurred speech may be experiencing a stroke, which is an emergency medical condition. When your partner is applying oxygen, it is most important for you to quickly assess the patient's vital signs and neurological status, which includes checking their level of consciousness, pupil reaction, and movement in all extremities. It is also critical to establish the onset time of the symptoms as it can influence treatment options. Coordinating rapid transport to a hospital that can provide acute stroke care is crucial.