In Igbo context,the word,'Olu na Igbo',is often used to mean Everybody but that word actually have origin which i personally do not know.
I therefore inquired and was informed that prior to British colonisation,that there are people who speak what is presently known as Igbo language but these set of people were classified as Ndi Olu instead of Ndi Igbo. They include people of Ogbaru,attani and the people along that axis.
How true is that?
Which people were the Olu people?

To the best of my knowledge, in the Anambra area, the term 'Olu' refers to the riverine peoples (Ogbaru, Onitsha, Ndoni, Oshimili, etc.) that live alongside the Niger and it's tributaries. These people are traditionally fishermen. The term 'Igbo' refers to inland peoples who are traditionally farmers.

Traditionally, in this part of Igboland, there were two main occupations from which communities could derive their sustenance: farming and fishing. Hence, the term 'Olu na Igbo' refers to both those that farm and those that fish: everybody.

...and the term 'Olu/Oru' was not confined to groups that spoke forms of Igbo or to groups from Anambra. The riverain Igala, especially those from the area around Ibaji, and Ijaw groups of the Central Delta were also historically referred to as Olu/Oru.

Among Igbo-speaking groups, communities in Ndokwa and Oshimili (Delta) and the Oguta and Egbema area (Imo/Rivers) were also referred to as Oru/Olu.

Basically, as Odumchi said, it was a term for people who lived near the Niger and its major tributaries south of Idah, but regardless of ethnicity. It has come to be closely associated with Igbo-speaking groups though.

Ejeem Olu.
Olu n'Igbo.

Morocco Asili 98: " Ndi ilo m jelu olu jee igbo".

"Olu na Igbo nine na-ekwu, emeka morocco na i ga adilili ndu".

' Olu na Igbo' from this morocco song seem to mean 'far and near' or everywhere in the first line.

In the second line, it seem to mean everybody.