Hydrangeas need lots of water!!  Remove as many unwanted leaves as possible and be sure to give the stem a cut on a 45 degree angle under water and consider cross cuts at the base of the stem/cutting up through its center about 0.5 inch up to increase surface area. Place the hydrangeas in a clean container with plenty of warm water (avoid metal containers as hydrangeas react negatively).  Allow hydrangeas to fully hydrate (2-3 days) before using floral foam/Oasis and ensure the foam/Oasis is at maximum water level at all times.  If your heads start to drop or droop, try the hot water trick; 1) recut stem as directed above 2) submerge the fresh cut in hot boiling water for 20-30 seconds 3) immediately place the stem back in clean warm water.  The blooms should revive back in a few hours.  This should unblock the stem and help the flow back up to the blooms.