I am Speed
Ability: Quick Feet
Item: Toxic Orb/Flame Orb
Flame Orb is never recommended on a Physical Sweeper that doesn't have Guts due to the Attack Drop.
As for Obstagoon, its typing is odd. Normal adds nothing to Dark defensively beyond upgrading its resistance to Ghost to an immunity, though the additional Normal STAB (especially with Guts + Facade) is welcome. Which is a shame, because Obstagoon's defenses are decent. Really that's Obstagoon in a nutshell, decent. Decent Defenses, decent Attack, decent Speed. It has the tools to perform any role except special sweeper, but is easily outclassed.
Additionally, in order to get to acceptable levels of Power, Obstagoon is heavily reliant on its ability, whether that is Guts or Defiant. Guts is generally preferred in Singles due to being able to reliably setup on your own, while Defiant actually has its use in doubles due to the number of Intimidates, Icy Winds, etc. flying around to easily nab a boost or two.
Mosh Pit
Obstagoon
Ability: Defiant
Item: Choice Scarf
-Knock Off
-Obstruct
-Parting Shot
-Close Combat/Ice Punch/Gunk Shot
Nature: Jolly (+SPE, -SATK)
EVs: 4 HP/252 ATK/252 SPE
Doubles Obstagoon is quite a different beast from Singles Obstagoon. Defiant combos well with Choice Scarf, allowing Obstagoon to patch up its Speed and Power to turn into quite the threat. Knock Off is a Great STAB that causes disruption by removing items which can be key to some strategies (i.e. Weakness Policy Tyranitar). Obstruct is Obstagoon's personal Protect that while not protecting against status moves punishes Predicted Fake Outs and Attacks by dropping the target's DEF. Yes you will have to switch out after an Obstruct, but that DEF drop can be key in nabbing that crucial KO. Parting Shot enables Obstagoon to pivot out while weakening the attacker's offenses, if you feel that Obstruct is not the right move for the situation. Coverage move is up to personal preference. Close Combat decimates opposing Dark types that resist Knock Off, Ice Punch maims the numerous Dragon and Flying types and Gunk Shot bins Fairy types including the ever useful Tapus.
Nature and EVs are the standard Sweeper Array, maximizing power and speed.
Other Options
Switcheroo -Tricking an Item is a great way to shutdown an opponent. Maybe even two depending on what item you get.
Seed Bomb -Hits Water, Rock and Ground types hard, but Obstagoon prefers the coverage from Close Combat and other moves more, not to mention Grass is resisted by a lot of types.