If you don't pay the overload cost of a spell, that spell will have a single target. If you pay the overload cost, the spell won't have any targets.

Because a spell with overload doesn't target when its overload cost is paid, it may affect permanents with hexproof or with protection from the appropriate color.

Note that if the spell with overload is dealing damage, protection from that spell's color will still prevent that damage.

Overload doesn't change when you can cast the spell.

Casting a spell with overload doesn't change that spell's mana cost. You just pay the overload cost instead.

Effects that cause you to pay more or less for a spell will cause you to pay that much more or less while casting it for its overload cost, too.

If you are instructed to cast a spell with overload "without paying its mana cost," you can't choose to pay its overload cost instead.

The set of creatures that gets +1/+0 from an overloaded Teleportal is determined as the spell resolves. Creatures you begin to control later in the turn won't get +1/+0. However, because the second part of Teleportal's effect doesn't change the characteristics of any permanents, the set of creatures that can't be blocked is constantly updated. Any creature you control at the moment blockers are chosen can't be blocked.