[Opportunity cost is] not infinite, but near infinite, yes. It's not merely my logic, it is a simple matter of observable fact. No matter what you do, there is a near infinity of things that you will not do at the same time...Let us say that (using Vox's example) we are going to a Clapton concert, but in order to do so, we must forgo a Dylan concert that we subjectively* value at $50. Our opportunity cost to go to Clapton is therefore the $50 in value we are giving up in order to do that. Clear enough?
The attempt to reduce this near infinity of choice to two choices is the Ricardian Vice. The attempt to quantify it with a dollar value is the Samuelsonian Vice. And the attempt to ignore the question posed and conflate the question with other definitions of opportunity cost is simply incorrect.
But to go to Clapton, we are also not going bowling (which we subjectively value at $20) nor are we playing Dragon Age ($30). By Vox's reckoning here our opportunity cost to go to Clapton has risen to $100 ($50 + 20 + $30). Since there is "a near infinity of things that you will not do at the same time," the opportunity cost of Clapton (or any other action) is near infinite once we add them all in.
Here's the booboo: It doesn't matter how many other choices there are, because you can only choose one. My choice is not Clapton OR (Dylan + bowling + Dragon Age), my choice is Clapton OR Dylan, Clapton OR bowling, Clapton OR Dragon Age. We MUST reduce the choice to a selection between Clapton and one other thing** because in reality we can go to Clapton or we can do one other thing. While I may be rejecting EACH option, I am not rejecting ALL options, because selecting all the options is not an option.
Opportunity cost is "The cost of an alternative that must be forgone in order to pursue a certain action. Put another way, the benefits you could have received by taking an alternative action." AN alternative. ONE single alternative. It is not based on the thing chosen (e.g. going to Clapton), but on the thing rejected (e.g. not going bowling). Therefore there are a million different opportunity costs for each choice, but they simply cannot be added together as Vox seems to have done to reach his "near infinity." He may be correct that opportunity cost is not really worth measuring, but he's certainly incorrect in measuring it that way.
* which is the only way to do it anyway. Ticket prices don't matter, pizza prices don't matter. We judge the experience that we are not doing to be worth $50, and that's its opportunity cost.
** Or between any 2 other things (e.g. bowling OR Dragon Age).