What does this mean on my 20 week scan sheet??.....?

Hi, I've just had my 20 week scan and the nurse said all is fine with baby (thankfully!) I just wondered if you could help on a couple of things. It says Placenta: posterior, and Presentation: breech, and also Site clear of OS. What is OS? Does the breech thing mean anything at this stage or should baby turn by the time I give birth anyway? and what does the posterior placenta mean? You help would be greatly appreciated. Thanks.