How does Double Strike work with Trample?
It's power and toughness never change. It's never 1/1 or 1/7 or 7/1. It's 7/7 throughout. It merely gains marked damage.

In the first Combat Damage Step, the Ogre assigns 7 damage. It will assign and deal 2 damage two each of the 2/2[1] (and kills them) and 1 to the player.

In the second Combat Damage Step, the Ogre assigns 7 damage. It will assign and deal 7 damage the player.

If the damage it did the second step was reduced by the damage it did in the first step, it would never deal any in the second as a creature always deals its power in damage.




  and if wrecking ogre is blocked by 2 5/5 creature?


Only creatures with First Strike or Double Strike participate in the First Strike combat damage step.

Assuming these 5/5 have neither,


First strike combat damage step: Ogre deals 5 to one 5/5 (and kills it) and 2 to the other (which survives with 2 damage).
Normal strike combat damage step: Ogre deals 3 to the remaining 5/5 (and kills it) and 4 to the defender. The 5/5 deals 5 to the Ogre (which survives with 5 damage).





It's possible to assign more, but there's rarely a reason to assign more than the minimum.