Mobil Corp. reported a $116 million loss for the third quarter yesterday as a result of a decision to set aside more than half a billion dollars from its earnings for a previously announced restructuring of its struggling Montgomery Ward & Co. retailing subsidiary.

Mobil, the nation's second-largest oil company after Exxon Corp., said that, without the one-time-only write-off, its earnings would have shot up almost 65 percent from a year earlier.

Elsewhere, No. 4, Amoco Corp., citing falling oil prices, said its net income fell 18 percent in the third quarter from the comparable 1984 period. Occidental Petroleum Corp. said its profits soared 150 percent, bolstered by a big tax break and the sale of some of its Colombian holdings.

Finally, Ashland Oil Inc., the nation's leading independent refiner of gasoline, reported that it completed its strongest fiscal year since 1980. In fiscal 1984, Ashland suffered a big loss as a result of a restructuring in which it announced plans to dispose of many of its non-oil businesses.

Mobil, based in New York, said its third-quarter net loss compared with a profit of $238 million (59 cents a share) a year earlier. Revenue fell 4 percent to $14.4 billion from $15 billion. The results were in line with analysts' expectations.

The loss was caused by a $508 million charge against earnings to cover the costs of restructuring Montgomery Ward.

Mobil said in May that it wanted to transform the 113-year-old retail chain into a smaller, profitable operation that eventually could stand on its own. One of the first moves was to close Ward's money-losing catalogue stores and related distribution centers.

Mobil has yet to disclose its plans, but has said Montgomery Ward could be sold or spun off into an independent company when the restructuring is completed.

Without the special charge against earnings, third-quarter profits would have totaled $392 million (97 cents), as higher gasoline prices in the United States and lower prices for crude oil helped improve profit margins from refining gasoline.

For the first nine months of the year, net income fell 37.5 percent to $615 million ($1.51) from $984 million ($2.42) a year earlier. Revenue dipped to $44.2 billion from $45 billion.

Without the Montgomery Ward restructuring, however, profits would have increased 14 percent to $1.12 billion ($2.76).

*Chicago-based Amoco Corp. said its profits fell to $490 million in the third quarter from $600 million a year earlier.

Earnings per share slipped 12.6 percent to $1.87 from $2.14. The decline was not as steep as the drop in net income because a stock buyback program reduced the number of shares outstanding.

Revenue nudged up to $7.2 billion from $7.1 billion.

Amoco said its operating earnings from oil and natural gas production fell 28.9 percent in the third quarter to $353 million from $496 million, mostly because of a decline in crude oil prices.

Amoco also said a decline in the value of the dollar from a year ago resulted in foreign exchange losses from operations abroad, but the company did not provide figures on the dollar's impact on earnings.

For the first nine months, earnings fell to $1.56 billion ($5.91) from $1.72 billion ($5.98). Revenue fell to $21.5 billion from $22 billion.

*No. 9, Occidental Petroleum Corp., which is based in Los Angeles, said its third-quarter earnings jumped to $381.7 million from $152.9 million a year earlier.

The improvement included a $180.9 million gain from federal income tax benefits carried forward from an earlier capital loss to offset gains this year. Also during the third quarter, Occidental sold half of its interest in a new, rich oil field in Colombia for $1 billion to the Royal Dutch-Shell Group of Companies.

Sales in the quarter fell 12.8 percent to $3.4 billion from $3.9 billion, reflecting a decline in oil prices.

For the first nine months, earnings rose 67 percent to $643.3 million from $384.5 million. Revenue fell 7 percent to $10.8 billion from $11.6 billion.

Earnings per share, which were reported on a fully diluted basis, rose to $3.04 in the third quarter from 90 cents a year earlier and climbed to $4.30 for the first nine months of the year from $1.89.

*Ashland Oil Inc. reported a profit of $46 million ($1.34 a share), on sales of $2.2 billion in its fiscal fourth quarter, which ended Sept. 30. Gains from its gasoline business led the improvement from a year earlier, when Ashland had a $241 million loss on sales of $2.3 billion.

The company, which is based in Ashland, Ky., said it earned $147 million ($4.12) in the full fiscal year, against a loss of $172 million a year earlier. Revenue fell 3.5 percent to $8.2 billion from $8.5 billion.

In the final quarter of fiscal 1984, Ashland took a one-time charge against earnings of $271 million for expected losses and write-offs related to the disposal of its non-oil businesses.

Without that charge, profits would have totaled $30 million (79 cents) in the final quarter of fiscal 1984 and $92 million ($2.13) for the full year.

John Hall, Ashland's chairman, said the divestiture program announced last year was virtually complete.