By default Notes window syntax set to Origin Rich Text by default. Right click window title and check Syntaxt node.

Right click the paragraph and choose Paragraph Styles. There are many preshipped styles e.g. Body Text, Bulleted List, Numbered List, Heading Left, Heading1, Heading2, Picture. Or user can define styles.

Use Tools: Text Style Manager to edit, create new styles and delete styles.

Close Origin, delete TextStyles.ini under User Files folder and restart Origin to reset to default settings.