What happens if 7 Soldiers block a Phyrexian Obliterator?
The Phyrexian Obliterator takes 7 damage.

Besides first strike and double strike (which create a separate combat damage phase for first-strike damage), all creatures exchange combat damage simultaneously. So the Phyrexian Obliterator takes all 7 points of damage even though it only has 5 toughness.

To add insult to injury, its triggers go on the stack after state-based actions are checked (meaning it and 5 Soldiers are dead), so you'll have to sacrifice 7 permanents in addition to the 5 tokens you lost blocking it.