How does portcullis interact with oath...
Can a player keep setting aside creatures due to portcullis, or does oath finish after 1 creature?
You Oath until you find a creature. That creature comes into play and triggers Portcullis. Portcullis' trigger resolves and checks the number of creatures and decides whether it needs to remove the new creature from the game or not.
So "No" you cannot keep setting aside creatures, Oath only triggers once and doesn't care what happens to the creature after it came into play.
If you have multiple Oaths you resolve them the same way as the first. All the Oaths go on the stack and you resolve them one at a time. Each Oath checks during resolution to make sure that the condition is still true, just like Portcullis will check each time.
Ok, does 0 damage attacks trigger damage related effects like hypnotic specter with weakness, or soltari visionary against an energy field???
Zero damage is considered the same thing as no damage. If you take zero damage from a Hypnotic Specter, you do not have to draw a card.
Note this distinction between Ophidian and Shadowmage Infiltrator. Contagion is good versus Infiltrator, not so good versus Phid.
That should read "you do not have to discard a card." Sorry.