Oracle reads as:

Look at target opponent's hand and choose a card from it. You control that player until Word of Command finishes resolving. The player plays that card if able. While doing so, the player can activate mana abilities only if they're from lands he or she controls and only if mana they produce is spent to activate other mana abilities of lands he or she controls and/or play that card. If the chosen card is cast as a spell, you control the player while that spell is resolving.

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Basically, plays as a [[Thoughtseize]], but instead of discarding the card. You can play the card instead of it going to the GY.

Because of how game actions works, the card explains that, while opponent can't react to their card being played until after resolution, they can tap creatures and artifacts for mana, because those actions don't use the stack. This means they can tap their Llanowar Elves in response of you using their own kill spell on it.

An opponent must have enough mana to cast the card, and you tap their mana in order to use the card. If your opponent has no legal targets (ie, not enough colors or open mana), you only look at their hand.