The building blocks of all were dust and ice grains circulating freely in the interstellar gas cloud that formed the solar system. These primordial materials are believed to have been reasonably well mixed in a very wide inner-planet region. The pioneer probes showed that today's totally dried out Venus has only a tiny quanity of water. Venus has only a hundred thousandth as much water as the Earth. The four Pioneer probe craft didn't find it in the atmosphere.
Many scientist believe that much of the missing oxygen is locked up in the planet's crust.
It has combined into such oxygen-rich minerals as hematite and magnetite. However, to get the crust to lock up this much oxygen, an efficient mechanism for remixing the planet's surface rocks is required. A few hundred million years of precipitation, erosion, and weathering produced by Venusian oceans may well be this mechanism. More recent work by Dr. Kevin Zahnle at NASA-Ames suggests that even larger oceans are possible. Then no trace of the original water would remain, and Earth-sized oceans are easier to explain.