This study employs a Randomized Controlled Trial (RCT) design. Patients diagnosed with oral cancer by the attending physician and who meet the inclusion criteria will be admitted to the study after receiving a full explanation of the study objectives, intervention methods, potential risks, and participant rights from the research team upon their stable transfer to the plastic surgery ward post-operation. Written consent will be obtained from the patient and/or their legally authorized representative after adequate informed consent has been provided, before they are enrolled in the study.

The study adopts a Randomized Controlled Trial (RCT) design. Patients diagnosed with oral cancer by the attending physician and who meet the inclusion criteria are enrolled in the study upon their stable transfer from the Intensive Care Unit (ICU) to the plastic surgery ward post-operation.

The research team will personally explain the study objectives, intervention methods, potential risks, and participant rights to the subjects (and/or their legally authorized representatives) and provide written documentation. After addressing all questions, informed consent is obtained, and the subject is enrolled in the study.

Subjects are then assigned via computer-generated randomization at a 1:1 ratio. The experimental group (18 individuals) receives hydrogen-rich water oral care. The hydrogen-rich water is delivered daily after its concentration is measured.

Oral integrity is assessed using the Oral Assessment Guide (OAG), which includes 6 items: lips, tongue, saliva, oral mucosa, gums, and teeth, scored from 1 to 3 points. The minimum score is 6, and the maximum score is 18.

Oral care frequency is based on the OAG score:

6-10 points: Oral care once per shift. 11-14 points: Oral care once every 4 hours. 15-18 points: Oral care once every 2 hours. Oral care involves using a sponge toothbrush dipped in hydrogen-rich water (concentration 1.1 mg/L) to gently brush the mucosa, maxilla/mandible (upper/lower jaws), and tongue. This is continuously performed for 7 days.

The control group (18 individuals) receives plain boiled water for oral care, with the same frequency as the experimental group. The research team will daily record the subjects' compliance and any adverse reactions.

During the intervention period, both groups receive identical postoperative care and health education in the plastic surgery ward.

The research team will collect the first sample (pre-test, T0) when the patient is transferred from the ICU to the plastic surgery ward, and the second sample on the 7th day post-operation (T1). Saliva samples will be collected at both time points to evaluate oral inflammatory markers (such as IL-1$\beta$, IL-6, IL-10, TNF-α) and the oral microbiome (analyzed by RT-PCR detection of the 16S rRNA gene sequence)

Inclusion Criteria:

• Diagnosed with oral cancer and receiving microsurgical free flap reconstruction.
• Age 18 years or older.
• Conscious and able to follow study instructions.
• Able to communicate without difficulty in Mandarin, Taiwanese, or through written communication.

Exclusion Criteria:

• Abnormal salivary gland function or severe xerostomia.
• Recent use of antibacterial mouthwash or antibiotic therapy that may affect oral microbiota.
• Presence of severe bacterial or viral oral infections, or significant periodontal disease.