when traveling with a lap baby, you do not need to advise delta in advance - just let them know when you check in (you probably will have checked bags anyway). most planes have 4 oxygen masks per row of 3 seats, but some (perhaps older 737s) have them only on one side of the plane.

three things to remember about car seats on planes:
1. you have to pay for the ticket, and delta offers no discount.
2. on some planes, the seat will only fit in the window seat.
3. monitor your reservation! on my last trip with my 3-yr-old, delta switched his seat without notice. i was lucky to find a row with enough empty seats 24 hours before the flight.