Here is an image showing Mt Everest and a Venus mountain compared to Olympus Mons on Mars.

 

However, I've always thought that if Mars once had water, then a great part of Olympus Mons would've been underwater. So it's kind of "cheating" to declare it the tallest mountain among our sun's planets.

 

Here is an image of Earth's Mauna Kea, in case you think the seafloor part of a mountain is used in the measurements. If that were the case, then Mauna Kea would be higher than Mt Everest.

 

So my question is, does anyone know the deal with Olympus Mons? When they measured it, was a pre-existing Martian ocean given consideration?

 

Or does that matter after the ocean is dry? For example, if Earth dried up, would scientists declare as the new tallest land mountain?

Mountains are mostly measured from sea level, it's pretty much the standard. Mt. Everest reaches higher altitudes than Mauna Kea and that's pretty much the point. Mountains erode, they grow and their surroundings are altered so it's sort of pointless to measure any other than the current situation. And anyway I haven't seen any estimates of there being an over 10km deep ocean on Mars.

we could take it as the variation from the average planetary radius. seeing as we're taking records from places without an ocean.

Wikipedia says the height of Olympus Mons is 27km above the mean surface level of Mars. So even if there were oceans covering most of the planet and it were measured from sea level, it would still be far taller than Everest, by any measure.

I think it's fair to say that it is cheating, considering that there is no water on Mars. Most mountains are measured from sea level from what I know. As there is no water on Mars, I think they just measure it from the mean land surface of Mars.

 

But then in my opinion, if you think about it, Olympus Mons is 27 kilometres from the MSL. Earth's tallest mountain measured from the base is Mauna Kea is 10km, Olympus Mons still beats, methinks

to baby astronaut

even if you measure the height of mount olympus mons from the seafloor it is still the highest mountain.for referrence see google

 

But if they find a still higher mountain they will declare that mountain as the highest(i used this line for your title)