Can Titania, Voice of Gaea be melded on the same turn that she is brought into play with Oath of Druids?

Question

If Oath of Druids brings Titania, Voice of Gaea into play under control of a player with Argoth, Sanctum of Nature in play and 4 lands in their graveyard, will Argoth and Titania meld into Titania, Gaea Incarnate on that turn?

Both the Oath of Druids and the melding are triggered at the beginning of the upkeep, so the underlying question is: Is the beginning of the upkeep a window of time where one thing can trigger after another and cause other beginning of the upkeep effects to trigger, or is it like a chime that happens once/turn and wouldn't effect something that wasn't there during the sole triggering?

Answer 1

No, Titania cannot meld during the upkeep when she enters the battlefield.

The beginning of the upkeep is a single event, and the only triggered abilities that trigger at that time are the ones that already existed as the upkeep started. Rule 503.1 says

503.1 The upkeep step has no turn-based actions. Once it begins, the active player gets priority. (See rule 117, “Timing and Priority.”)

• 503.1a Any abilities that triggered during the untap step and any abilities that triggered at the beginning of the upkeep are put onto the stack before the active player gets priority; the order in which they triggered doesn’t matter. (See rule 603, “Handling Triggered Abilities.”)

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It is possible to meld Titania on the same turn that she enters, using Paradox Haze.

Answer 2

Both the Oath of Druids and the melding are triggered at the beginning of the upkeep

Yes, both are triggered at the begin of upkeep. Once they resolve, it's not "beginning" of upkeep anymore.

Is the beginning of the upkeep a window of time where one thing can trigger after another

Certainly that's possible, but you're not asking about one thing triggering another thing, you're asking about something triggering, then resolving, then that resolution being considered in an "at beginning of upkeep" trigger.

Suppose you have a card that says "Whenever a creature dies, create a Spirit token" and another that says "Whenever a creature dies, if you control a Spirit, gain one life". A creature dies, and at its moment of death, you don't control any Spirits. Do you gain a life? No; the second trigger checks the board state at the moment of death. It doesn't wait for every other trigger to resolve, and then trigger. "At the beginning of upkeep" is the same: the trigger sees the board state at the moment that it becomes upkeep.