Volcarona
Heavy-Duty Boots is a staple on any Volcarona moveset since it is 4x weak to Stealth Rock, so my first suggestion is to change Life Orb to Heavy-Duty-Boots. Also, I would recommend changing Morning Sun to Psychic so you that you can beat Toxapex, which resists Bug Buzz and Flamethrower. If you think you don't need that, you can just change it to Roost instead since it has more PP making it better than Morning Sun.
Volcarona @ Heavy-Duty Boots
Ability: Flame Body
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Flamethrower
- Bug Buzz
- Psychic/Roost
- Quiver Dance
I will have all of the proposed changes pasted at the end of this post, as well.
Tyranitar
Tyranitar's viability is on the decline with the addition of a lot of better heavy hitters coming from the new DLC. Urshifu Single-Strike (the dark type one) is proving to be an incredibly good physical attacker right now. The Rock typing doesn't do much for your team and you would still have a Dark type as a Psychic immunity should you need it. Urshifu hits a lot harder with it's signature Dark-type move having effectively 120 base power, and it is also much faster than Tyranitar. I would swap Tyranitar for Urshifu because of this. There isn't much of a reason not to.
→ 
Urshifu @ Choice Band
Ability: Unseen Fist
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant/Jolly Nature
- Close Combat
- Wicked Blow
- Poison Jab/Sucker Punch
- U-turn
There are few that can switch into both Close Combat and Wicked Blow with the Choice Band boost in attack. Fairy types like Clefable even get 2HKO'd unless they invest EVs in Defense. Poison Jab will easily take care of them, though. Sucker Punch is a good option as you currently don't have any priority moves on your team. If you go for that, make sure to only use it if you really need to since Choice locking into Sucker Punch can let your opponent set up on you freely. U-turn is great as Urshifu will often be forcing out switches, so you can capitalize on that by U-turning on incoming checks.
Cloyster
Cloyster doesn't have much longevity as a defensive water type. It's only healing will be from leftovers since it doesn't have access to any recovery moves and being weak to Stealth Rock heavily cripples its ability to switch in. Personally, I think Toxapex is a much better fit here as it can easily heal with Recover or just heal by switching out with its Regenerator ability.
→ 
Toxapex @ Black Sludge
Ability: Regenerator
EVs: 248 HP / 252 Def / 4 SpD / 4 Spe
Bold Nature
- Scald
- Haze/Knock Off
- Toxic
- Recover
You could run Knock Off since you don't have a Knock Off user on your team, but Haze is pretty useful in neutralizing set-up sweepers. Its really up to you. You could also run Rocky Helmet instead of Black Sludge if you want, but I think the extra healing is really helpful in being able to switch into opposing banded Urshifu more frequently. The rest is self-explanatory.
Blissey
Blissey is a good pick here, though there are just a few things I would change. For starters, there is really no point in having Special Defense EVs on Blissey since it can take special attacks very well without them at all. It's much better to help her on the physical side of things. You don't have a Stealth Rock user on your team which is pretty essential, so I would definitely add that. Also, I would change Wish to Soft-Boiled as it is just a more reliable self-healing move since it only takes one turn. The last thing I would change is to add Teleport. It's incredibly useful on a pokemon like Blissey that would completely take away all your momentum without it. It can get you a free switch-in to Urshifu to start dealing huge damage, or to Volcarona to start setting up for example. Your final moveset should end up looking something like this:
Blissey @ Heavy-Duty Boots/Leftovers
Ability: Natural Cure
EVs: 252 HP / 252 Def / 4 SpD
Bold Nature
- Seismic Toss
- Soft-Boiled
- Teleport
- Stealth Rock
One more thing. I personally really like Heavy-Duty Boots on Blissey, but this is completely optional. I think being able to switch in without taking entry hazard damage is a huge boon for Blissey. You can just stick with Leftovers if you want, though.
Jirachi
I know I have already suggested to change 2 pokemon, but I promise this will be the last one. A steel type is always important to have on a team, and while Jirachi is a good one, you currently have no ground-type immunity AND no defog user. Naturally, Corviknight can patch that up nicely. You could use Skarmory instead, but I think Corviknight makes for a better Defogger while Skarmory is better at Spikes stacking.
→ 
Corviknight @ Leftovers
Ability: Pressure
EVs: 248 HP / 252 Def / 8 SpD
Impish Nature
- Body Press
- U-turn
- Defog
- Roost
You could run 140 SpD EVs to avoid the OHKO from +2 Life Orb Alakazam's Focus Blast, but your next member should do a fine job of keeping Alakazam from getting out of hand.
Dragapult
Well, first of all, Baton Pass isn't even allowed in OU, so that is gonna have to go. Thunder Wave isn't a move you're going to be able to use that often with Dragapult's relatively weak defenses, so I would take that off as well. I don't think you can afford to run mixed offenses since Dragapult isn't that strong on either side and it kind of needs to be focused on either full physical or full special attacks. Let's just start from scratch and go with this set:
Dragapult @ Leftovers
Ability: Infiltrator
EVs: 252 Atk / 4 Def / 252 Spe
Adamant Nature
- Dragon Darts
- Phantom Force
- Dragon Dance
- Substitute
This set can easily come in on passive pokemon that can't touch it like opposing Blissey or Chanseys and set up a Substitute to block statuses, then proceed to set up freely. Alternatively, you can pivot in with Blissey's Teleport or U-turn from Urshifu and Corviknight. Dragapult is fast enough to outspeed Alakazam without needing to a Dragon Dance boost or a Jolly nature, which is quite nice.
Volcarona @ Heavy-Duty Boots
Ability: Flame Body
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Flamethrower
- Bug Buzz
- Psychic
- Quiver Dance
Urshifu @ Choice Band
Ability: Unseen Fist
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Close Combat
- Wicked Blow
- Poison Jab
- U-turn
Toxapex @ Black Sludge
Ability: Regenerator
EVs: 248 HP / 252 Def / 4 SpD / 4 Spe
Bold Nature
- Scald
- Knock Off
- Toxic
- Recover
Blissey @ Heavy-Duty Boots
Ability: Natural Cure
EVs: 252 HP / 252 Def / 4 SpD
Bold Nature
- Seismic Toss
- Soft-Boiled
- Teleport
- Stealth Rock
Corviknight @ Leftovers
Ability: Pressure
EVs: 248 HP / 252 Def / 8 SpD
Impish Nature
- Body Press
- U-turn
- Defog
- Roost
Dragapult @ Leftovers
Ability: Infiltrator
EVs: 252 Atk / 4 Def / 252 Spe
Adamant Nature
- Dragon Darts
- Phantom Force
- Dragon Dance
- Substitute
Believe it or not, you had the foundation of a great team for your first time. I know it seemed like I changed a lot, but I tried to pick similar things to your original choices that were just better options. I think the end product turned out great and I hope this helped.
