Occidental Petroleum Corp., the nation's 12th-largest oil company, become the first major international oil firm to report third-quarter earnings when it said yesterday that its profits dropped 28.2 percent in the period from their levels a year earlier. But nine-month earnings set a record.

The company attributed the decline primarily to a drop in its oil and gas operations' third-quarter earnings to $129.9 million from $231.6 million a year before due to "lower production and [profit] margins in Libya and the imposition of taxes on Occidental's share of production in Peru."

Occidental said it earned $133.7 million ($1.62 a share) in the third quarter, down from $186.1 million ($2.44) a year earlier.Sales rose to $3 billion from $2.6 billion.

Nine-month earnings were a record $572.7 million ($7.19), up 58.2 percent from $374.8 million ($4.85) a year earlier. Sales rose to $9.3 billion from $6.7 billion.

Occidental said nine-month results were trimmed by a sharp drop in profits at its coal division, where "higher labor costs that could not be recovered because of a weak domestic market," and "transportation bottlenecks" held earnings to $5.2 million against $35.3 million a year earlier. In the third quarter, the coal division lost $1.8 million against a profit of $8.4 million a year before, Occidental said.

But "higher margins from the North Sea and in Libya in the first and second quarters" pushed Occidental's oil and gas earnings to $537.4 million in the first nine months from, $452.2 million a year before, Occidental said.

Per-share earnings of RCA Corp. fell to 84 cents in the third quarter from 86 cents a year earlier in spite of a substantial gain in actual net because of higher preferred dividend requirements.

This resulted from the issuance of a new class of preferred stock in connection with the acquisition of C.I.T. Financial Corp.

Net income was $80.4 million on revenues of $1.99 billion compared with $66 million a year ago on revenues of $1.83 billion.

Nine-months net imcome included a $14.5 million gain on special items, including the sale of the Random House publishing business and the proceeds of insurance on the space satellite lost last year. But the earnings also were cut by expenses related to cancellation of the Moscow Olympic games broadcasts.

Last year's nine-month profits also were increased $28.8 million by a British tax change and a gain on the sale of the RCA Alaska Communications business.

Chairman Edgar Griffiths said profit of the National Broadcasting Co. was up only slightly even though NBC has reached the No. 1 spot in prime-time ratings but that RCA's domestic and global communications business, its consumer products operations, the other manufacturing operations and the Hertz Automobile rental business all did quite well.

Merrill Lynch & Co., benefiting from soaring volume on Wall Street, posted a 33.8 percent gain in third-quarter profits to $50.2 million ($1.37 a share) from $37.5 million ($1.03) in the same period of 1979. Revenues rose to $710.1 million from $528.7 million.

"Although every major revenue category contributed to the record results, the sharpest gains occurred in the traditional securities and financing services," said a statement issued by Chairman Donald T. Regan and President Roger E. Birk.

"Reflecting the surge in securities volume and our increasing share of these markets, commission revenue soared by 53 percent during the third quarter," they said.

Nine-month profits were up 69.5 percent to $151 million ($4.14), larger than any of its previous annual profit figures, from $89.1 million ($2.44). Revenues rose to $2.17 billion from $1.46 billion.

Another major broker, Shearson Leob Rhodes, reported profits for the quarter, the first in its fiscal year, rose 140 percent $21.8 million ($3.34 a share) from $9.05 million ($1.25) a year earlier. Revenues rose to $209.1 million from $110.6 million.

PPG Industries Inc. reported yesterday that its third-quarter net imcome fell to $47.8 million ($1.44 a share) from $56.3 million ($1.74) in the same period last year. Sales declined to $765.3 million from $784.3 million.

Nine-month net income dropped to $142.2 million ($4.32) from $163.8 million ($5.09). Sales increased to $2.32 billion from $2.29 billion.

Irving Bank Corp., the bank holding company whose principal subsidiary is Irving Trust Co., the nation's 16th-largest bank, reported net income of $20.4 million ($2.31 a share) in the third quarter, a 15.9 percent gain over its 1979 third-quarter earnings of $17.6 million ($1.99).

Income before securities transactions was $20.52 million ($2.33), a 15.3 percent increase from the $17.8 million ($2.02) before securities transactions last year.

Nine-month net income rose 32.1 percent to $64.54 million ($7.33) from $48.86 million ($5.55), while income before securities transactions was $64.7 million ($7.35) compared with $49.56 million ($5.63).