Brig. Basilio Olara Okello, the new leader of Uganda, is a career military man believed to be in his mid-fifties whose career has blossomed since the overthrow of dictator Idi Amin in 1979.

According to observers here, Okello fled Uganda in 1971 when Amin came to power and remained in Tanzania until 1979. He then returned to the country with troops that had been dispatched by Tanzanian President Julius Nyerere to oust Amin.

Okello had been the commander of the Ugandan Army's Central Brigade, which includes the capital city of Kampala, when Obote returned to power in a disputed election in 1980.

According to reports reaching here from Kampala, he was transferred to the Northern Brigade about two years ago after a dispute with his superiors.

In a country where tribal loyalties and hatreds play a key role in propping up and overthrowing national leaders, it is significant that Okello is a member of the Acholi tribe of northern Uganda, a tribe with many men in leadership positions in the Ugandan Army. Reports this weekend from Kampala indicate that Okello has aligned himself with another Acholi, Lt. Col. Tito Okello, the commander of the Army under Obote. Tito Okello is not believed to be related to Basilio Okello.

Obote, a member of the Langi tribe, is believed to have angered many Acholi military officers, including Basilio Okello, when he appointed a fellow Langi, Smith Opon Acak, to be Army chief of staff two years ago. Opon Acak replaced another Langi.

Relations between the two tribes, which traditionally have been dominant in the Army despite their relatively small size, have deteriorated considerably during the past few months, with several gunfights reported in the capital in recent weeks between units composed largely of Langis and Acholis.

Okello is described as a career soldier who rose through the ranks to become a brigade commander in the north. He reportedly was born in Gulu, in the north, and joined the King's African Rifles in 1962, just before Uganda became independent that year, according to a Briton interviewed by The Associated Press.

Okello, who addressed the nation briefly today on Radio Uganda, is a Roman Catholic said to favor reconciliation between Uganda's rival tribes. He also is reported to favor rapprochement with an insurgency movement.