If you're about to encounter an Oni please don't read.

Last session I made a horror themed session revolving around an Oni, I'm using a custom stat-block and I spent the session spooking the players and actually got some really good reactions out of everybody. The session ended with the players staying in a halfling's farmhouse just for him to activate gaseous form and leave the farmhouse and return two of the party's horses that vanished previously, although the horses are horribly spooked and hurt.

The Oni is logically next to the horses invisible if he just led them over, combat was about to start and I called the end of the session. I made the encounter with the party at level 5 and they just leveled up to level 6.

What worries me is that they've made a group chat to discuss plans to kill the monster, they don't know the initiative order yet.

So far my plans are to charm their eldritch knight, he has disadvantage on saves against the Oni. The party believe that this character is the target, they have somehow convinced each-other that the Oni can only shape-change into somebody he has ate, and this character is a lord's son who would be a good person to impersonate. The real reason the oni has showed them attention is that they have the lowest wisdom and he plans to charm them.

The party is a fighter, cleric, warlock and two wizards, they frequently use dispel magic and counterspell against Spellcasters. My current plan is charm the fighter, cone of cold the rest, focusing on the wizards. The remaining players are a forge cleric and Eladrin Archfey warlock. I know the forge cleric's character has become boring for the players as he's the father figure and the fun police of a chaotic party. The Eladrin has a ring of resistance to cold damage, and is immune to sleep and being charmed, making her a good counter to the Oni. If a player drops below 30hp, I'm gonna hit them with sleep if possible as his sleep is 4th level and 11d8, and that has 2/3 chance of success.

With their planning, they are prepared to steamroll the villain and all their work will be for little payoff, as the fight wouldn't be well, fought, what can I bring to the game to even the odds, either clever tactics or if needed, bringing more enemies to the battle, but not just meat bags to distract.

For one, I'd say good planning leading to steamrolling a villain can be very fun and satisfying, and its not something you should be afraid of.

For two, if the party has him outnumbered 5-to-1, and he's lost a chance at surprising them, he may just leave just leave. Onis are wise enough to know not to go into a losing fight. If you're really worried, give him a lair with a couple lair actions and/or servants, and let him lead the party back there, so he can fight them on his terms. Make a dungeon out of it to wear down their resources a bit before they fight him.

Unless you gave the eladrin a special trait, they are not immune to charm effects.  Elves just have advantage on the saves against them.

And as the person above stated, if the Oni is alone the party with obliterate it, give it some minions.

First and foremost, this is why I don't allow PC only group chats in my games. Obviously I can't actually "prohibit" it, but I talk to my players about the fact that I view it to be kind of messed up. My game isn't "DM vs PCs", and furthermore, if your characters don't have time to strategize, neither do you. If you bust out moves in combat related to monster weaknesses that there's no way your character would know, I'm gonna let you do it, but I'm also gonna call you out in front of the entire group about how there's no way you'd know to do that and if it happens again I might not let it fly. 

Now on to your encounter. It sounds like you've slightly painted yourself into a corner since the stage for combat is already set, so calling in extra monsters might not be the way to go. I'd instead  plan for ways to make it a challenging fight by upping the Oni's abilities to test your party's resources and for creating contingencies allowing the Oni to escape. Then it knows your party's strength and you can set up an encounter down the line where it's more prepared, plus the party gains a repeating  villain that they want to chase and destroy. With that said:

I'd go for something like 3 legendary actions per round to take immediately after a PC ends their turn, and 1 legendary resistance per day. For the legendary actions, I'd probably go claw attack (costs 1 action) and Cast Darkness (costs 2 actions). This gives your Oni an extra 3 claw attacks per round, upping the danger level in terms of damage in a big way. It also allows a backdoor escape plan. Oni are smart monsters. An INT of 14 implies that they can plan, adapt tactics to new situations, and know when to flee when the odds are stacked against them. With all that in mind, I'd probably do something likle this:

First, Cone of Cold from invisibility to start the fight. This spooks the horses and I'd probably create some stampede-related environmental hazard for the first round, forcing a DEX save and having PCs pushed 10 feet/knocked prone/trampled for maybe 1d6 bludgeoning damage on a failure. Next, the Oni's plan A for combat has to be using its fly speed to hover 10 feet in the air using its glaive, taking your fighter somewhat out of the equation. If the party starts to deal a lot of damage, it will change tactics to focus on whoever is dealing the most, getting within 5 feet of that PC so that it can use its glaive on its turn while staying in range for its legendary claw attacks. Once the Oni is starting to get hurt (around 60-70%%), it's probably starting to think about escaping, even if it's not ready to flee just yet. It can use those 2 legendary actions immediately after the attack that brings it down to 60% (or whatever HP target you set) to cast darkness centered on itself, forcing your casters to counterspell and/or dispel. If they do, it still has one legendary action left to swipe with its claw at anyone in range that turn. At the top of the round, it regains 10HP and flies over to whichever caster hit it with dispel or counterspell, using its turn to unload on that PC with the glaive and getting within 5 feet of the caster to set up it's legendary claw attack for later, repeating the legendary actions for darkness immediately after the next PCs turn. You can also use the Oni's turn to move within 5' of a caster and then cast Darkness as a regular action, forcing them to either make their next attack against the Oni with disadvantage allowing for a legendary claw attack (with disadvantage), move out of the darkness and prompt an opportunity attack with the glaive (also unfortunately with disadvantage), or counterspell/dispel, effectively forcing them to burn two 3rd level slots per round on simply countering darkness. Rinse and repeat until your party is out of dispel/counterspell, which should be 6 castings between the two wizards, 8 if the warlock knows it as well, and hopefully they've used one to un-charm the fighter as well.

Once you get through a round where the darkness remains up by the time it gets to its turn, it can cast invisibility and use its fly speed to get out of there, maybe heading up to the roof to regroup while its regenerate slowly brings it back up to full health and it can fight your party while they have diminished spell slots, or just have it bail. That kind of depends on what its motivations are with regards to your campaign.

I dunno, that's just a first shot and I might've missed something,  but that's about how I'd approach it. 

Well, unless they're using Telepathic Bond, the ogre heard all of their plans.

You can assume the oni has prepared so.....before turning ivisible it assumed the form of one of the party, surprise round for him is to move behind the party and cast darkness over them. Party I would assume will then dispel it to reveal two identical party members,

Or, have him accompanied by a coulple of Changelings (both lvl 3 one wizard and one bard, their his spies he sends into settlements), no real damaging spells but plenty of debuffs and things to hamper the party, hold person, sleep, suggestion, Create bonfire for some battlefield hazards (or a dsctraction if they want to set a buildingon fire), Colour Spray or Blindness/Deafness and Silence. They'll have sufficiently low HP as to not be overwheleming and be able to provide some distractions.

If it's just a matter of making the fight more tough, you can add minions, the Oni might have brought disguised help (changelings, or monsters waiting just outside). If there's a lore reason for the oni to have prepared this halfling's farmhouse in advance, make one or more of the things there mimics :) or maybe one of the horses that was brought back is actually the mimic, and one horse was eaten already?

I wouldn't like using the players' plans against them. That seems like it would just be frustrating - basically tells the players "don't bother planning, plans get automatically countered anyway." If they're reasonable plans, it's ok for them to work - it feels pretty awesome when you make elaborate plans and because of some reasonable rolls, they actually *work*. It doesn't feel anticlimactic at all.

I think multiple spellcasters with counterspell, against *one* spellcaster, is always going to be reasonably easy fight. The action economy just works out that way - oni spends his turn casting something, wizard spends his *reaction* to counterspell, oni basically doesn't get to do anything. Still worth trying for high-level AOE spells in case the wizard doesn't make his save, but might not be worth trying for single-target low-level spells like Charm Person.

The oni can also do things like collapse the farmhouse on them maybe?

Have the oni start combat from a flying position above the party, so they can land next to whoever the weakest person is instead of making it easy for the tank to tank.