Oasis started in Uyuni as a receptive operator running the salt flat and the whole south-west of Potosí department, and grew into three offices — the original one on Ferroviaria beside the Hotel Julia, one in Sucre on Aniceto Arce, and one on Sagárnaga in La Paz. That matters if you want to book the salt flat before you are standing in Uyuni at seven in the morning with every agency on the street quoting you a different number.
They run regular departures from Uyuni and from La Paz, which is the useful bit: the La Paz departure takes the altiplano road down rather than leaving you to find the overnight bus.
Each office has its own mobile. The Uyuni one is the WhatsApp, and the button on the site is labelled "Cotizar por WhatsApp" — get a quote on WhatsApp — which tells you how they expect to be contacted.