The expression aboard is older than on board, however, the two expressions can be used interchangeably to mean on/onto a ship, plane, train, or bus:
- Jack was already aboard/on board when he remembered that he’d left his suit at home.
- The plane crashed, but everyone on board/aboard survived.
- Are all the passengers aboard/on board?
- This is your captain speaking. Welcome aboard/on board of this British Airways flight.
Welcome on board/aboard can also be used figuratively when welcoming a new member of a team:
- Welcome on board/aboard, James always speaks very highly of you.
- Welcome, Janine. It’s good to have you on board/aboard!
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