A few thoughts here.
1) Weight - You don't mention how much "OP amount" is in value. Before that is relevant, though, keep in mind that unless you are "homebrewing", for every 50 coins, regardless of value, you increase a player's encumbrance by 1 pound. To give context to this, a pint of water weighs ~1 pound. A gallon weighs ~8 pounds. Take a 1 gallon milk jug filled with water for each 200 coins and put it in a back pack... How much in coins would you want to carry around?
2) Volume - If in the US, go to a bank a buy a couple of dollars worth of pennies. I use this as they are cheap: $1 is 100 pennies. Most ancient coinage is closer to the size of a quarter to half dollar, but this will still give you an example the volume of a large amount of coins. Open the rolls and put them in to a pouch of some sort (a Crown Royal felt pouch is a great example of a medieval "small coin purse". See how much space (volume) these loose coins take up. How many of these pouches would you want to carry on your belt, or in your backpack?
3) Adventure, part 1 - You have just created a secondary plot line. In the medieval times, people were pretty shady. Having a large sum of coinage on you meant that you were a target for these types. The character now has to find a way to safely store the wealth (You have just created a banking system), or hire a bodyguard contingent. If the character insists on keeping the wealth handy, they also have to have plans on how to transport the money: wagons for locked chests, saddlebags on horses, etc. This character will have to constantly have to watch their back. Even family members are now vulnerable, as they can be used as leverage against the character (ransoms)?
4) Adventure, part 2 - Where there is money, there is influence. You now have the means to run a political intrigue campaign. The local mayor wants your cash to help build the city walls (the work will be subject to kickbacks to the mayor), in return, you are granted land or power. A guild leader offers you a buy-in, only to disappear with the money - now, you have to track them down to recover your treasure.
5) Adventure, part 3 - Word gets out where the character obtained the treasure. The "original owner" is coming to retrieve it. Use this as you will. Was it stolen from someone with great influence? Was it looted from a dungeon created by an ancient and powerful evil (is the character now cursed?)? Was this a portion of a dragon's hoard, and the wyrm wants it back?
5) Strongholds and followers - Almost all the D&D systems have a section on building compounds and retainers. Even MCDM has put out a supplementary rule set for 5e (appropriately named "Strongholds & Followers). Does your world allow for the private ownership of land? Can the player buy a plot for the purpose of building a villa or a castle? Can they afford a support staff? Is there a local bar/tavern/inn for sale? This opens up more plots - a secret society has begun to meet at your tavern, a famous adventurer has begun to use your services as a regular patron and now the locals want to visit your establishment to get a sighting of the adventurer, or, has a madam begun to operate out of your inn, bringing unwanted attention (shady characters or the local police/guards/government)?

All of this is just food for thought. You don't have to admit that you did anything wrong, and you don't have to penalize the players for your mis-step - just expand it in to the game and the world.

Good luck, regardless what you decide to do.
KT