Litton Industries Inc. said yesterday that earnings were off 36 percent for its second quarter and first half because of weak oil prices, intense competition in the microwave-oven business and heavy interest expenses.

For the three months that ended Jan. 31, the Beverly Hills, Calif., high-technology concern netted $47 million ($1.68 a share) on sales of $1.17 billion compared with earnings a year earlier of $74.2 million ($1.74) on sales of $1.12 billion.

For the first half of its year, Litton earned $90.7 million ($3.24) on revenue of $2.27 billion, in contrast to 1984, when it earned $141.9 million ($3.33).

The relatively small change in per-share earnings reflects Litton's $1.3 billion repurchase last year of 35.8 percent of the company's stock. It now has about 27.8 million shares outstanding, compared with 42.4 million last year.

*Western Union Corp. reported that it lost $367.2 million in 1985.

The company said the large loss, equal to $16.07 a share, occurred largely because of a fourth-quarter $300 million write-down of outdated switching and transmission equipment.

The $367.2 million loss compares with a 1984 loss of $58.4 million ($3.04 a share). Revenue was $1.082 billion, down from $1.134 billion in 1984.

The fourth-quarter charge resulted in a loss for the period of $335.7 million ($14.02), up from a loss of $62.2 million ($2.81) in the comparable period of 1984.