The 100-year-old Coca-Cola Co. yesterday reported a 15.8 percent increase in earnings a share for 1985, with a whopping 20.6 percent increase in the fourth quarter -- a gain that may reflect the return of the original-formula Coke.

Meanwhile, Geo. A. Hormel & Co. and International Harvester Co. reported higher profits for their first quarters, and Phibro-Salomon Inc. posted increased profits for the fourth-quarter of 1985 and the entire year.

Coca-Cola Co.'s net income was $722.30 million ($5.51 a share) for 1985, compared with $628.81 million ($4.76) the year before. Fourth-quarter earnings were $1.45 a share, 45 percent higher than 1984's quarterly earnings.

During 1985, the firm abandoned original formula Coke in favor of a new Coke. The change backfired, however, with consumers mounting an aggressive campaign for the original formula. Coca-Cola brought back the original formula under the Coca-Cola Classic label and created a domestic sales spurt in the fourth quarter.

"The strength of our soft-drink business, combined with leadership in the citrus . . . drink business and a rapidly growing entertainment sector, position us very well . . . ," said Robert C. Goizueta, the firm's chairman and chief executive officer.

*Geo. A. Hormel & Co. reported an increase in sales and improved earnings in the first quarter, despite an unresolved strike by meat packers.

Net sales for the quarter ended Jan. 25 were $480.81 million, a large increase over the 13-week fiscal 1985 first-quarter sales of $357.15 million, the company said.

Earnings rose 7.4 percent to $8.9 million (46 cents a share), compared with a year earlier net income of $8.23 million (43 cents).

Local P-9 of the United Food and Commercial Workers Union struck the Austin, Minn., plant six months ago, seeking the $10.69 an hour they earned 16 months ago, before the company cut pay to $8.25 to meet competition in the meat-packing industry. 17.

*International Harvester Co. posted a $28 million profit (18 cents a share) during the first quarter of its new fiscal year on sale of $789 million, the company announced yesterday.

This was the fourth consecutive profitable quarter since Harvester shed its farm-implement business in January 1985 to concentrate on building heavy trucks.

For the first three months of 1985, Harvester had a net loss of $534 million on sales of $840 million.

*Phibro-Salomon Inc. reported increased fourth quarter and annual profits for 1985, crediting the performance of its Salomon Bros. and Phibro Energy units. The commodities and financial services holding company reported net income of $132 million (86 cents a share) for the quarter ended Dec. 31. That compared with a loss of $130 million in the period a year earlier.

For the quarter, total revenue for the corporation was $8.59 billion, compared with $7.88 billion in the final three months of 1984.

For the year, Phibro-Salomon had net earnings of $557 million ($3.60), compared with $212 million ($1.41) in 1984.