Chili oil is used to make spicy sauce for fusion type sushi rolls.
Chili oil is made by frying red chili peppers in oil to infuse the flavors. It is originally a Chinese condiment and used for many dishes, especially in the Schezwan cuisine.
When choosing it for making spicy sauce for sushi, it is important to choose one that uses a light flavored oil (rapeseed, or canola) with red chili peppers as the sole ingredient.
Some Chinese oils are made with peanut oil and may sometimes add scallion and ginger while making them. While this beautifully matches with Chinese cuisine, the aromas are too strong to make sauce for sushi.
A list of essential Sushi condiments is available here.