Atletico agreed an £81.5m deal to sign Manchester City forward Julian Alvarez on Monday, and selling Omorodion will help raise funds to buy the Argentine.
It is understood personal terms have now been agreed with Alvarez, who will join for an initial £64.4m, with a further £17.1m in potential add-ons.
In a separate deal, Chelsea midfielder Conor Gallagher has agreed to join Atletico for £33m.
Omorodion, 20, joined Atletico last summer – days after making his La Liga debut for Granada, when he scored against the Madrid side.
Atletico immediately sent Omorodion to Alaves on loan, and he scored eight goals in 35 appearances last season.
He is currently at the Olympics and has scored once in Spain’s run to the final.
So far this summer, Chelsea have spent about £115m on midfielders Kiernan Dewsbury-Hall, Renato Veiga and Omari Kellyman, plus goalkeeper Filip Jorgensen, defenders Tosin Adarabioyo and Caleb Wiley, winger Estevao Willian and striker Marc Guiu.
They have sold defenders Ian Maatsen and Lewis Hall and forward Omari Hutchinson, while Hakim Ziyech, Malang Sarr and Thiago Silva have left on free transfers and Gallagher is nearing his switch to Spain.
The future of Romelu Lukaku is uncertain, with the wantaway Belgium striker having spent the past two years on loan in Italy.