Artifice Expert. Oslow, Tinkerer has advantage on Intelligence (History), Intelligence (Arcana) checks and Wisdom (Survival) checks related to explosives, black powder, alchemy, the elements, and the Construct creature type.

Goggles of Arcana. Oslow can see and otherwise sense magical auras of spells and other magical effects as per the detect magic spell. Creatures armed with magical objects have disadvantage on Dexterity (Stealth) checks made to Hide. 

Healing Mend. If Oslow, Tinkerer casts his mending cantrip on a Construct or a Humanoid (Warforged), the target is healed for 1d4+1 hit points.

Spellcasting. Oslow, Tinkerer is a 17th-level spellcaster. His spellcasting ability is Intelligence (spell save DC 18, +11 to hit with spell attacks). He has the following artificer spells prepared:

Cantrips (at will): acid splash*, guidance, mage hand, mending, poison spray*
1st-level (4 slots): absorb elements, cure wounds, faerie fire, snare
2nd-level (3 slots): aid, heat metal, lesser restoration
3rd-level (3 slots): haste, tiny servant
4th-level (3 slots): fabricate, stoneskin, stone shape
5th-level (1 slot): greater restoration, skill empowerment

*Cantrip appears in Actions

Multiattack. Oslow, Tinkerer makes two attacks Dual-Barrel Pistol attacks, or casts two of his Cantrips. 

Acid Splash (Cantrip). Ranged Spell Attack: range 60 ft., one creature in range, or two creatures in range that are within 5 ft. of each other. Each creature within range must succeed on a DC 18 Dexterity saving throw or take 14 (4d6) acid damage.

Poison Spray (Cantrip). Ranged Spell Attack: range 10 ft., one creature in range. The target must succeed on a DC 18 Constitution saving throw or take 26 (4d12) poison damage.

Dual-Barrel Pistol. Ranged Weapon Attack: +4 to hit, range 30/90 ft., one target. Hit: 9 (2d6 + 2) piercing damage.

Alchemists Fire (Recharge 5-6). Oslow, Tinkerer throws a vial filled with a glowing orange and green liquid at a point within 30 ft. of him. Each creature within a 10 ft. radius sphere of the targeted point must make a DC 13 Dexterity saving throw or take 1d6 fire damage at the end of each of its turns. It can end this effect by using it's action to make a DC 10 Dexterity check to extinguish the flames. Additionally, anything within 10 ft. of the point not being worn or carried is ignited.

Oslow, Tinkerer can take 3 legendary actions, choosing from the options below. Only 1 legendary action can be used at a time and only at the end of another creatures turn. Oslow, Tinkerer regains spent legendary action at the start of his turn.

Alchemist's Fire. Oslow recharges his Alchemist's Fire and uses it.
Spellcasting (Costs 2 Actions). Oslow uses his Spellcasting feature, casting a spell of up to 3rd-level or lower (including Cantrips) from his spell list that takes 1 action to cast. 
Construct Reinforcements (Costs 3 Actions). Oslow whistles to the winds, calling his signature Clockworks on standby to arrive to combat. When this action is taken, Oslow can choose to summon either 2d8 clockwork bronze scouts* or 1d6 clockwork stone defenders*, which all appear in the closest unoccupied spaces to Oslow. Regardless of type, all constructs summoned this way are under his control and take their turns after his in the initiative order. If Oslow falls unconscious or dies, all constructs summoned by this effect are simultaneously incapacitated, and cannot awake until Oslow recovers.

*As found in Monsters of The Multiverse.