What is water made mostly of?

It should have been so simple...
I asked Google a single, simple, question.
Turns out, that's how all the fun rabbit holes are stumbled upon.
The question I asked was as follows.
What is water made mostly of?
The answer as it turns out was not so simple. You see, it really depends on which race your putting water into.
If it's a shear numbers race then hydrogen certainly wins. Water is made of a single oxygen atom bound to two hydrogen atoms. So there it is, for all to read. Water is made of mostly hydrogen atoms. Yet, if you were trying to find out what water is mostly made of in regards to simple mass than water by weight is made mostly of oxygen because oxygen atoms are sixteen times heavier than hydrogen atoms. Confused? Your not alone. Me too.
So, to simplify, a single molecule of water has twice as many atoms of hydrogen than oxygen atoms. Yet it is outweighed by oxygen. Which means that water is made mostly of oxygen. Or at least that's what we are lead to believe...
So, this raises another interesting question. (This sometimes happens and is generally a consequence of asking a good question in the first place).
The question is as follows...
Weight vs Single constituents? What's more important in water?
When Google answered my original question, it directed me to sources that indicated that water was mostly made up of oxygen due to oxygens atomic mass. But this simply cannot be true. When it's a numbers game, like it most certainly is when asking a question about what something is mostly made of, the answer should be directed to the sources indicating that water is made mostly of hydrogen atoms. Because when we are talking about things in this universe, we should refer to things that are relatable to this universe and its atomic scale. Put simply, we don't normally count things by weight, we count them by individual constituents. To say that water is made mostly of oxygen is like saying that a zoo that has two elephants and one hundred birds mostly has elephants because the birds are so small. It simply doesn't make any sense. Yet, when talking about how much something is made of, we are being taught to forget about the numbers and just replace mass as the single most important value. Perhaps this is because mass is an integral part of one of the most valued formula's of physics. What is this formula, I hear you asking? Well, that would be Einstein's famous E = MC2. So, perhaps this is why, when talking about what something is mostly made of, we talk about it in relation to its mass and not its individual constituents? The fact that water is made mostly of oxygen by mass means that if we were to somehow chop off one of the hydrogen atoms, the water molecule should be just fine, that is, the molecule should still be a water molecule. Right? Well, actually, no. After chopping off one of the hydrogen atoms you would have created a hydroxide ion. While this is one of my favourites types of ions it is simply no longer a water molecule. So, it turns out that the numbers of the individual constituents are important after all. Sure, by mass oxygen rules the roost in water but without hydrogen you simply wouldn't have water. For this reason and until further notice this post belongs in both the Oxygen Journal, as well as the Hydrogen Journal. But, for the time being you will only find it posted under the Oxygen Journal, because, you know, Google...
Told you it wasn't a simple answer. Good luck wrapping your noggin around that explanation.