If my opponent has an Oath in play, and he gives me a token via orchard, but I lava dart it, and none of us have any creatures in play after the dart resolves, can he give me another token during his upkeep and then stack the Oath so that he gets Akroma?
Timing comes into play here. Here is the layman's description of the way Oath and other "At the beginning of your upkeep" effects work.
The golden rule of Oath is that it checks the creature count TWICE at most... and Once at least. Let’s look at the turn:
Untap -> No players get priority
Upkeep -> First ALL Triggers that read "At the beginning of your upkeep" Go onto the stack. Those that are conditional ... Like oath, check the condition to see if they infact go on the stack or not. NO PLAYER has gotten priority to make an action this turn before the oath trigger counts creatures. Once all the "beginning of upkeep" triggers are placed on the stack then the Active Player (the player whose turn it is) get priority to make an action. Then Non-Active player will get a chance before the first trigger resolves. Why does all this matter?
The creature count MUST be in the Active Players Favor in order to put the Oath trigger on the stack. If the oath trigger is not on the stack, then it can never resolve. So for example: It is the very beginning of my turn, I control 0 creatures and 2 tapped forbidden orchard, you control 0 creatures. I untap my orchards and move to my upkeep. At this moment Oath tallies up the creatures and sees 0 vrs 0 therefore it never goes on the stack. I am never given the opportunity to Tap an Orchard before this First check is made. Even though I have ability to make the creature count 0 vrs 2, The oath will not trigger this turn, because It never went on the stack.
So suppose you used your lava dart at the END of your turn, then your opponent could NOT oath.
Let’s switch it up a bit. Let say that at the beginning of your turn your opponent played Orim's Chant and then gave you a creature (making it impossible for you to dart it durring your turn). The turn passes, I untap, Oath checks and sees 0 vrs 1 then the trigger goes on the stack. Now I have priority, All looks good to me, so I pass it over to you. You now have a chance to play the dart you could not play at your End of Turn. So you dart the creature and Pass priority back to me. Now Oath is on the stack ... BUT it also checks AGAIN when it attempts to resolve. So If I allowed the trigger to "resolve" it would recheck creatures and see 0 vrs 0 and effectively NOT oath up a creature.
So what can I do? well oath is still on the stack (going to 0 vr 0 did not remove oath from the stack) and it is my priority to make an action. So I will tap my now untapped orchard to give you a new creature before the oath trigger resolves. If you allow the Oath trigger to resolve, the creature total is back to 0 vr 1. And as Oath resolves it will see the 0 vr 1 and continue to oath a creature into play (even though for a little while the creature total was 0 vr 0). The fact that the creature total went to 0 vr 0 for a moment did NOT remove the Oath trigger from the stack. Remember that Oath only checks at the begining of the upkeep, and if sucessfull, then it will check only once more as it is resolving.
Now its back to you with the creature total 0 vr 1, and all my orchard are tapped, Oath has NOT yet resolved. Now lets take a look at the Dart's ability to Flashback. In this example you infact CAN fizzle the oath. Let’s see how that would play out...
Remember we started this turn at 0 vr 1 (bc of the orims chant durring your turn, and I have one orchard in play). Oath goes on the stack, before it resolves you dart your creature 0 vr 0. After the dart from hand resolves, I used my untapped orchard to give you another creature bringin us to 0 vr 1 again. Then you flash back dart targeting the new creature makeing it 0 vr 0. Now I can't do anything else, and you choose not to do anything else... so Oath "resolves" and checks the creature count - it sees 0 vr 0 and does NOT put a creature into play. As I said at the beginning of this Its all about Oath checking at most twice durring any one upkeep. Here is a little logic chart:
Before Players get priority If My Creatures equal or greater than Your creatures ==> Oath does not trigger. and never will.
Else [ My Creatures < Yours ] Continue
Oath Goes on the stack, Both players may play instants and instant abilities. This can adjust creature totals
When Oath Resolves if My creatures < Yours ===> I -MAY- Oath a creature into play
Else [ My creatures >= Your creatures ] ==> Oath does not bring a creature into play.
As Far as Gaea's is concerned. Oath must fully resolve before Gaea's Blessing triggers. This means that you reveal cards until you hit a creature, or there are no more cards to reveal (ie they had no creatures). THEN you put all non-creatures that were revealed into the graveyard. At this point Gaea's blessing triggers and goes on the stack. Oath has now fully resolved (weather it put a creature in play or not). And both players again get priority. At some point, assuming no one stifles, The gaea's blessing trigger will resolve and shuffle that players graveyard into what remains of the library (even if it is a 0 card library). And play continues as normal. Keep in mind that Gaea's blessing does not happen immediately when it triggers, both players are given priority to take action before the Blessing trigger resolves. So you can win the game with ancestral recall even if your opponent flipped several Gaea's blessings into the yard. You will get a chance to cast ancestral recall before any and all Gaea's blessings actually resolve. If they have 0 cards in the library when you resolve that ancestral recall targeting them ... they loose the game.
So the short answer to your questions is:
#1 - If you cast Dart during your turn, then NO. If you cast dart durring their upkeep then Yes (unless you cast it twice and they only have one orchard)
#2 - No, the blessing will trigger after oath has fully resolved ... not durring oath's resolution.
