I am 27 week pregnant and my last ultrasound reports says "low lying placenta partially covering the os" should i be worried?

Depends: You did not say if the placenta was anterior or on top or postetior along the back of the uterus. On any event since it partially cover the opening by which the baby is delivered, it needs to be watched. Usually the growth of the baby and increased size of the uterus causes the placenta to pull back from the opening not to cause problem. Doctor should watch