IIRC, the extra oxygen mask is a 4th mask for a group of 3 seats (ET or domestic config) but on BA not all rows have the 4th mask. But in CE where only 2 of the 3 seats are sold there shouldn't be a problem because all groups of seats should have a 3rd mask that could be used by the infant due to the convertible seating that will allow 3 people to sit.

However, the seat selection tool might not realise this!