Gasoline supplies at most of the 1,500 service stations in the Washington area will increase markedly in August, oil industry spokesmen said yesterday.

Four major oil companies that supply most stations here - Exxon, Gulf, Mobil and Texaco - will increase August supplies from 2 to 8 percent over July, but stations still will receive 10 to 15 percent less than they were a year ago.

While spokesmen were optimistic that the Washington area and the nation will not see long gasoline lines again this summer or fall, they warned that it could happen.

"It would be disastrous if the American public saw no lines as a signal to go back to their old habits of using all the gas that they want;" Exxon spokesman Pat O'Connor said.

While gasoline lines began disappearing three weeks ago in the Washington area, there were reports of occasional lines this week in the Ocean City resort area - something that had not happened before.

Industry analysts said vacation spots such as Ocean City may suffer gasoline shortages in August because the government has channeled more gasoline to metropolitan areas and because people who put off vacations earlier this summer may decide to take them now.

Even with the lines virtually gone in the Washington area, officials said they have no intention of ending soon the odd-even rationing and minimum gasoline purchasing plans in effect since June 20.

"We're not going to life odd-even because it's working," said a spokesman for Maryland Gov. Harry Hughes.

The District of Columbia's energy chief, Chuck Clinton, said it would be "foolhardy" to lift the odd-even rules "until we can be sure ue won't fall back into a more serious [gasoline supply] problem."

Spokesman for Mobil and Texaco said yesterday that increases in world crude oil supplies accounted for the improvement in deliveries to their stations.

In early June, Saudi Arabia began producing an additional million barrels a day of crude oil, adding about 2 percent to the free world's daily supply.

"That's not much, but you've to to remember that a few percent one way or the other can make the difference between gas lines and price wars," said an industry analyst.

The August supply announcements were as follows:

- Exxon, with 450 stations here, will increase August supplies to 85 percent of year-ago levels from 83 percent in July.

- Gulf (150 stations), from 75 to 77 percent.

- Mobile (110 stations, from 82 to 87 percent.

- Amoco (174 stations) has not announced an exact August figure, but a spokesman said yesterday, "I'm sure there will be more gasoline available than there was this month."

- Shell (165 stations), remains the same at 70 percent.

- Sunoco (110 stations), remains the same at 75 percent.

The increases could bring 1,000 to 8,000 additional gallons in August to each of the stations affected, according to industry analysts. The average station here pumps 50,000 to 100,000 gallons a month, they said.

There was a similar supply increase to the Washington area from June to July, which analysts said helped account for the disappearance of lines here.

That increase is difficult to quantify, but according to estimated federal figures, the District received nearly 2 million more gallons of gasoline in July than in June - a 12 percent increase but still 4 percent less than it received in July last year.

The situation was further eased because Virginia and Maryland diverted emergency reserve gasoline into the Washington suburbs.

Virginia gave the suburbs here 2 million gallons in June and increased that to 4 million gallons in July, officials said. It is not known what the August figure will be.

At the same time these changes were easing the gasoline lines here, the Department of Energy made rule changes in its complicated gasoline allocation system in an effort to divert some gasoline away from rural areas to the cities.

The effects of the changes may be felt in August, analysts said.

Public officials and oil industry representatives interviewed last week said they were optimistic about gasoline and heating oil supplies for the next few months, but they warned that tight oil supplies will remain a chronic problem for America in the months and years ahead.

"Unless the supply of gas takes a marked turn for the worse, I don't think we'll have lines again," said Mark Edmond, editor of The Lundberg Letter, a respected petroleum marketing weekly.

"I would look for a pretty good August, and more people on vacation than thought they would. I'm driving out to Colorado and I didn't plan on it."

Oil industry spokesmen said that despite the greater availability of gasoline, the companies ought to be able to meet Persident Carter's goal for storing 240 million barrels of home heating oil by October.

A cold winter, however, could upset the delicate balance of supply and demand, they said.

American Petroleum Institute statistician Edward Murphy said that the world is on "the razor's edge" of worldwide supply and demand. An international incident in an oil-producing area, or a collective decision by Americans to return to their cars, he said, could bring on a crisis again.

Motorists must continue driving about 10 percent less in order to prevent a crisis, analysts said. Motorists, rather than truckers, farmers, defense agencies and those who heat their homes with oil, must bear the brunt of the shortage under federal rules.

The American gasoline crunch began this spring and was initially brought on by a cutoff of oil production by Iran, a major producer. The oil companies had low stocks and were unprepared to fill the gap.

The members of the Organization of Petroleum Exporting Countries were also unwilling to fill the gap, and have increased their prices 50 percent since last December.

The oil companies and service station owners seized on the shortage to raise their margins to the maximum, so that near dollar-a-gallon prices are now commonplace her and elsewhere in the nation.

While the price increase has been painful for consumers, analysts say it was a factor in curtailing demand and bringing an end to the gasoline lines.

The gasoline lines began in April in California and then spread to other parts of the country as federal allocation rules for scarce supplies went into effect and distorted allocation patterns.

Gasoline lines began in the Washington area in early May and lasted seven weeks before the odd-even plan, begun June 20, took hold. CAPTION: Table, Weekend Gasoline Guide, The Washington Post