As trading resumes this Monday, a significant sell-off is dominating the Australian equity market. Shares listed on the ASX are experiencing a sharp decline in today's session. The S&P/ASX 200 Index has fallen a substantial 3.64%, marking its most severe drop in years and bringing the index down to just above 8,500 points.

This follows the index's close at 8,851 points last week. The surge in oil prices is a key factor behind this dramatic downturn.

Investors began the day to find that Brent crude oil prices have surged from approximately $82 per barrel at the end of last week to well over $108 per barrel. This marks the first time oil has exceeded $100 per barrel since the initial Russian invasion of Ukraine in 2022.

The consequences of such a rapid and significant price increase are profound.

This surge highlights structural issues within the oil market, which is unsurprising given the known causes. As investors are aware, the US-Iran conflict has led to the effective closure of the Strait of Hormuz, a critical passageway that typically handles about 20% of global oil shipments.

Additionally, there have been multiple attacks on refineries and other key infrastructure in the Middle East's Gulf region since the war began. The duration of this new reality remains uncertain.

These combined factors have thrown oil markets into disarray, resulting in the substantial price increase observed today.

How Elevated Oil Prices Negatively Affect ASX Companies

Focusing on the impact of $100 oil on ASX-listed companies reveals a broad negative effect.

As noted, the market is declining sharply today. Nearly every sector is witnessing heavy selling, with energy stocks being the notable exception.

Why does an increase in oil prices seemingly harm the valuations of diverse companies from Qantas Airways Ltd to Commonwealth Bank of Australia and Woolworths Group Ltd?

The reason is that oil is a fundamental input cost for many businesses on the ASX. Its influence permeates the entire economy.

For Qantas, fuel is one of the airline's largest fixed expenses. Aircraft require jet fuel, which has now become significantly more expensive, contributing to the airline's more than 5% drop today.

For Woolworths, the expenses associated with transporting goods from suppliers to distribution centers and then to supermarkets have also risen. The company faces the difficult choice of either absorbing these higher costs or passing them on to consumers, presenting two challenging scenarios for shareholders.

Even companies not directly using oil to provide goods or services are affected. For instance, Commonwealth Bank is not a major consumer of oil. However, its performance is closely tied to the overall health of the economy.

Sustained high oil prices would likely hinder economic growth and fuel inflation. Money spent on oil, petrol, or diesel is money not spent elsewhere in the economy. This is unfavorable news for Commonwealth Bank and nearly every other non-energy company on the ASX.

Therefore, $100 oil poses a significant challenge for most ASX-listed companies and, by extension, for many Australians. It is understandable why the market is experiencing such a pronounced decline today. Investors will be watching closely to see how the situation develops throughout the week.