To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Orysa's first ability). The mana value of the spell is determined by only its mana cost, no matter what the total cost to cast that spell was.

The first step of casting a spell is to move it to the stack. If this causes the total toughness of creatures you control to change (perhaps because you control a creature whose toughness is determined by the number of cards in your hand), that new toughness will be used to determine whether or not the cost reduction from Orysa's first ability applies.

Once you determine the cost to cast Orysa, you may activate mana abilities to pay that cost. If the total toughness of creatures you control changes while activating mana abilities, the cost to cast Orysa remains what you previously determined.