This is one of those things I didn't think of until I was midway through my first flight with baby. That said, it's a crucial thing to know.
Oxygen: Every row of seats has an extra oxygen mask in case one is inoperable or there is a lap infant in the row (that's you!). So, if you're in a row of three seats, four oxygen masks will drop down in the event of an emergency.
Life vests: According to a 2007 memo from the Federal Aviation Administration, "all cabin occupants, including in-lap children, must have an individual flotation means available for use." But, since in-lap children can be spread throughout the aircraft, the question is where are these extra flotation devices?
In every flight I've been on, there was an overhead bin near the back of the plane that was marked to include infant life vests. My suggestion is to read the safety card in your seat pocket or ask a flight attendant so you are well aware where they're stowed in your aircraft.