Assuming that you meant ORAS OU when you said OR/AS, I am going to rate your team. Overall the team layout is quite fine, but some of the moves need work.
EDIT
Since you said that this may possibly be a VGC team, I am going to slash Protect on some movesets, and slash Healer with Natural Cure on Blissey. Protect is one of the best moves in Doubles that can buy that one turn for your other Pokemon to kill the threat, to block Fake out. I wont recommend Toxic if it is Doubles due to the fast-paced nature of the game. Using a weather Pokemon in Doubles is good as the 8 turns of Rain buys you quite a lot of turns to decide the game due to how fast-paced the Doubles metagame is.
Onto the rate!
Mega Swampert is a great pokemon in general, but you are better off abusing Mega Swampert's much better Attack (Base 150) compared to its Special Attack (Base 95...) Change his nature to Adamant and an EV spread of 100 HP/252 Atk/156 Spe to allow Mega Swampert to hit 215 speed, which outspeeds Mega Sceptile and Mega Beedrill in the rain.
He will be your main sweeper, so I will not put Stealth Rock on him, I will rather run him as an All-Out Attacker.
Here's a set:
![enter image description here][1]
Swampert @ Swampertite
Ability: Damp ===> Swift Swim
EVs: 100 HP/ 252 Atk/156 Spe
Adamant Nature
-Waterfall
-Earthquake
-Ice Punch
-Superpower/Rain Dance
Waterfall is superior to Water Pulse as it is a physical Water type move with better base power and a decent 20% chance to Flinch the opponent. Works quite well with Swift Swim. Earthquake is Earthquake. Ice Punch is mostly to hit Dragons and Grass types that wall its dual STABS like Dragonite. The only reason I put Superpower in is to smack Ferrothorn harder than any other move, and it can be interchanged with Rain Dance if you do not like the Atk/Def drop.
There really isn't a point to put full investment in Speed for Eelektross as its base 50 Speed is atrocious even with investment. I would give Eelektross an Assault Vest and give it full investment in HP to allow it to take hits better.
![enter image description here][2]
Eelektross @ Assault Vest
Ability: Levitate
EVs: 252 HP/4 Atk/252 SpAtk
Quiet Nature
-Volt Switch
-Flamethrower
-Giga Drain
-Knock Off
A slow Volt Switch is a good way to pivot Pokemon in safely without having them switch into strong attack moves, and overall gets STAB and is a great scouting move. Flamethrower to fry Ferrothorn (Its super annoying to this team) and Giga Drain to get recovery. Knock Off is a useful move in general that removes items, which will cause the opponent to think twice about sending in their Eviolite Chansey or Scarfed Pokemon with the risk that they lose their item.
Cleric Blissey is not bad, but some of the moves need a relook.
![enter image description here][3]
Blissey @ Leftovers
Ability: Healer/Natural Cure
EVs: 4 HP/252 Def/252 SpDef
Bold Nature
-Soft-boiled
-Heal Bell
-Toxic/Thunder Wave/Protect
-Seismic Toss
Assuming this is Singles, Natural Cure is superior to Healer as Healer only works in Doubles with an adjacent teammate. This allows Blissey to switch into Toxic, Thunder Wave, Will-O-Wisp, Sleep Powder... The list goes on. All it has to do is switch out and its Status Condition is cured, allowing her to act as an absorber of status. I changed her nature to Bold so she can take physical hits a little better while sacrificing its unimportant Attack stat (Base 10). Soft-boiled is a mandatory move to allow Blissey to keep herself healthy throughout the match. Toxic is her best way of crippling the opposition or you can use Thunder Wave instead, depending on which is more important to your team. Seismic Toss is her most reliable way of doing damage, and Blissey should not be staying in on Fighting types as it will probably get one-shotted by a Physical Fighting move anyway. Leave your teammates to deal with the Fighting types, that's what they are for.
The Haxorus set looks good! That set is missing Earthquake. Good, old, reliable, powerful Earthquake.
![enter image description here][4]
Haxorus @ Lum Berry
Ability: Mold Breaker
EVs: 4 HP/252 Atk/252 Spe
Adamant Nature
-Dragon Claw/Outrage
-Earthquake
-Poison Jab
-Dragon Dance
I gave Haxorus a Lum Berry to give it more setup opportunities. This allows you to get a free Dragon Dance on random status moves thrown at you and then proceed to obliterate them with a coverage move. With Lum Berry, Outrage also becomes an option as once troublesome Fairies and Steels are gone and your Lum Berry is still intact, you can feel free to click Outrage and witness the apocalypse that follows. I changed its Ability to Mold Breaker to complement Earthquake, this allows it to use Earthquake on Pokemon with Levitate like Gengar and the omnipresent Rotom-Wash. Earthquake is Earthquake, it destroys Steel types and have never done its job better.
Take this Rotom-Wash:
+1 252+ Atk Mold Breaker Haxorus Earthquake vs. 248 HP / 216+ Def Rotom-W: 278-328 (91.7 - 108.2%) -- guaranteed OHKO after Stealth Rock
Spiritomb seems somewhat irrelevant, as a defensive Ghost it is completely outclassed by Mega-Sableye, and its role in the team is questionable. The team in general looks weak to Mega Lopunny which can High Jump Kick your whole team without repercussions (with Scrappy, it hits Spiritomb for supereffective damage.)
252 Atk Mega Lopunny High Jump Kick vs. 0 HP / 200 Def Spiritomb: 344-408 (142.7 - 169.2%) -- guaranteed OHKO
I would put Cofagrigus in the team instead to nullify its ability with Mummy once it takes a hit, and is a great switch into Physical attacks in general.
Heres a set:
![enter image description here][5]
Cofagrigus @ Rocky Helmet/Leftovers
Ability: Mummy
EVs: 248 HP/252 Def/8 SpDef
Bold Nature
-Shadow Ball
-Will 'O' Wisp
-Pain Split
-Protect/Nasty Plot
252 Atk Mega Lopunny High Jump Kick vs. 248 HP / 252+ Def Cofagrigus: 121-144 (37.9 - 45.1%) -- guaranteed 3HKO after Leftovers recovery
And it loses Scrappy to Mummy after that, thus it is forced out. Shadow Ball is a STAB move to prevent it from being completely Taunt fodder, Will 'O' Wisp to burn Physical Attackers and halve their attack stats, Pain Split is Cofagrigus semi-reliable recovery, and finally Nasty Plot is to become an instant threat.
In terms of a sixth Pokemon to suggest for your team, I would recommend Drizzle Politoed if you are going to run Mega Swampert. Being able to set up rain, this makes the team much more threatening as Mega Swampert can ditch Rain Dance for a coverage move.
![enter image description here][6]
Politoed @ Damp Rock
Ability: Drizzle
EVs: 252 HP/252 Def/4 SpDef
Bold Nature
-Scald
-Encore
-Protect/Toxic
-Ice Beam
In terms of the threatlist to this team, this team will face a lot of problems facing Mega Lopunny as it 2HKOes your entire team bar Cofagrigus with High Jump Kick. You have no resistance to Dragon moves, this makes Lum Berry Dragonite somewhat of a threat as it can set up on Blissey and Eelektross and proceed to deal huge damage to this team. Perhaps I may switch Eelektross out for a Fairy-type Pokemon to have a switch-in for Dragon moves and Mega Lop to an extent.
Anyways, hope I helped!
[1]: http://www.pokestadium.com/assets/img/sprites/main-series/6/x-y/260.gif
[2]: http://www.pokestadium.com/assets/img/sprites/main-series/6/x-y/604.gif
[3]: http://www.pokestadium.com/assets/img/sprites/main-series/6/x-y/242.gif
[4]: http://www.pokestadium.com/assets/img/sprites/main-series/6/x-y/612.gif
[5]: http://www.pokestadium.com/assets/img/sprites/main-series/6/x-y/563.gif
[6]: http://www.pokestadium.com/assets/img/sprites/main-series/6/x-y/186-f.gif
