He died Sunday afternoon at the main hospital of the capital Niamey, the Social Democratic Party said via Whatsapp.
The country has recorded 736 confirmed cases of COVID-19 with 35 deaths, the World Health Organisation said.
President Mahamadou Issoufou said he learned the news of Omar’s death with a “heavy heart”.
Omar, born January 1, 1965, in the south-central town of Tesker, had led the PSD, which is allied with Issoufou’s Party for Democracy and Socialism.
Issoufou’s labour minister since 2017, Omar began his political life in 1999 under president Mamadou Tandja, serving in parliament or in various ministerial roles in the poor Sahel nation.
Omar strongly supported a constitutional amendment that allowed Tandja to prolong his second term by three years in 2009, but a military junta overthrew Tandja the following February.
Before he created the PSD, Omar was part of the Rally for Democracy and Progress, the party of Tandja’s predecessor Ibrahim Bare Mainassara, who was assassinated in 1999 by members of his own bodyguard.