It marks the first nationwide walkout since 1980 and impacts plants that together account for more than 10% of US refining capacity.
The United Steelworkers Union (USW) began the strike on Sunday, after their current contract expired and no deal was reached despite five proposals.
The USW said it "had no choice".
"This industry is the richest in the world and can afford to make the changes we offered in bargaining," USW International Vice President of Administration Tom Conway said in a statement.
"The problem is that oil companies are too greedy to make a positive change in the workplace and they continue to value production and profit over health and safety, workers and the community."
Royal Dutch Shell, the lead industry negotiator, said it "hopes to resume negotiations as early as possible".
The move comes at a tough time for oil companies, which have been cutting costs and reining in spending following a collapse in crude prices.
A large part of this is due to an increase in oil volumes extracted from shale formations, adding to a global supply glut.
USW said it represents workers at 65 US refineries that produce approximately 64% of the oil in the US.
The union has been renegotiating a three-year national contract since 21 January. The latest offer was the fifth proposal rejected by the union.
It wants to double the size of the annual pay increases from the previous agreement, increased healthcare coverage and reduced use of non-union contract workers.
Shell, Marathon Petroleum and LyondellBasell will see their facilities affected but have since implemented strike contingency plans.
The last nationwide work stoppage in 1980 lasted three months.
US union leaders have launched a large-scale strike at nine refineries after failing to agree on a new national contract with major oil companies.