I don’t often see a fundamental Vim feature that’s new to me¹, but today I learned about o_v from a comment on /r/vim. From motion.txt’s section about v:

When used after an operator, before the motion command: Force the operator to work characterwise, also when the motion is linewise. If the motion was linewise, it will become exclusive. If the motion already was characterwise, toggle inclusive/exclusive. This can be used to make an exclusive motion inclusive and an inclusive motion exclusive.

So what does this mean? Well, vim motions are either inclusive or exclusive. This dictates whether the character/line motion moves to should be affected by the preceding operator or not. The b motion (back [count] words) is exclusive, so if we have the text foo̲bar (the cursor is on the second ‘o’) db would result in o̲bar since the operator d is applied to the range [foo). If we would like it to include the second ‘o’ as well, we can use our new friend v to create the command dvb, which turns foo̲bar into b̲ar Antother – perhaps more useful example – is if you want to delete from the current column in a line, to the position just below the cursor (in the following line), you could use dvj to turn the linewise motion j into a characterwise motion.

I honestly don’t know if this will be useful, but I’m excited to discover such a fundamental feature nonetheless.