One of the most important artists of his generation, he obtained a diploma in art and design from the Buruburu Institute of Fine Art and went on to work full time as a painter at Kuona Trust Museum Art Studio.
Best known for two series: one exploring tribal mythologies from the Luo, Kikuyu and Taita peoples; the other, his Masquerade Series, examining identity and the artifice employed in disguising true intent - one of which attracted the then highest price ever paid for a Kenyan painting at Bonhams Auction House in London. He says of this iconic Masquerade series; “For the simple need to look better than they actually are, people go to strange lengths to change themselves... a complete make-over.”
Okello's works were shown at Pop Up Africa (GAFRA) in Cork Street, London in 2014/15. He has also exhibited in the Netherlands, France, India and at the Heong Gallery at Downing College, Cambridge, UK in 2017.
Along with I&M Bank his works are held in the collections of international accounting firm Price Waterhouse Coopers and the East African Visual Arts Trust.
He lives and works in Nairobi.