If you give Ochran Assassin trample, does it destroy all creatures my opponent controls?
No, trample has no effect on how an attacking creature deals damage to defending creatures.
With or without trample, an attacking Ochran Assassin can destroy any number of blocking creatures, up to its power. So if the Assassin has 3 power, it can destroy up to 3 creatures. This is because Deathtouch only has an effect if at least 1 damage is dealt, and any amount of damage with Deathtouch is lethal. So the Assassin can assign its total damage as power, split into one per blocking creature, and each creature it deals damage to that way is destroyed.
The only effect trample has is to allow you to assign excess damage that is not needed to destroy any creatures to the defending player. For example, if the Assassin has 5 power and the defending player only controls 3 creatures that can block, the Assassin can damage and kill all 3 creatures and also deal 2 damage to the defending player. But if the Assassin only has 1 power, it can still only destroy 1 creature.