H2O
Katherin Omak

The answers to the clues below will have something in common. Wordplay is fair play.  

1. Fateful day for Mr. Polo
2. Foes of a big oil company
3. Impressionist in a disco
4. Nutty place of song in Scotland

HINT:
The title is a hint.

Answers:

1. Ides of Marco
2. Arco enemies
3. Vincent van Gogo 
4. Loco Lomond

Each answer has an H replaced by an O.