vs. Protoss

Early scouting and the right unit composition can help you defend against a possible Zealot Rush. A well executed Zealot Rush can break a Zerg player attempting this build unless they have their defenses set well in advance. 

vs. Terran

Similar to your matchup against Protoss opponents, you will need good scouting to see an early Marine rush coming before it reaches your base. Surviving early aggression while continuingly pumping our Drones is your path to a mid to late-game victory.

vs. Zerg

Often ZvZ matchups come down to which player has the stronger macro game and the 15-Hatch Opening puts you in a very strong position to win the war of economies.

vs. Protoss

The 15-Hatch Opening is extremely weak against a Zealot Rush and you will be at a great disadvantage on maps with short rush distances. If a Zealot Rush is spotted, you will need to build Spine Crawlers and Zerglings with a transition into Roaches as soon as safely can.

If your scouts see a 4-Gate build from your Protoss opponent, you should have some time to build up your defenses before the expected attack. Spine Crawlers, Roaches & Zerglings should be sufficient to hold off a 4-Gate attack, but you will need to find a balance between your defense and your economy. Not enough defensive units and you can lose your expansion or possibly the game, but too many defensive units and you will be at an economic disadvantage going into the mid-game.

vs. Terran

Your biggest early game fear is a Marine/Bunker rush that will put your expansion in jeopardy. Good scouting on your opponent’s base as well as around your expansion can help you spot an early Bunker going down or a potential Marine Rush. Spine Crawlers and a mass of Zerglings will help you stave off a Marine rush, but an overreaction to the attack by making too many Zerglings, will cause your economy to suffer.

You will want to transition into Zergling/Banelings against Marine/Marauder and eventually Mutalisks to contain and harass your opponent as long as possible to maintain your economic advantage. 

vs. Zerg

If your scouting finds that your Zerg opponent has not taken their expansion, you will need to prepare for an early attack. Your scouting will tell you if it will be a Zergling/Baneling attack or purely Zerglings.

If an early Spawning Pool is found by your scouts, you will have to place a Spawning Pool of your own with the expectation of an early Zergling attack from your Zerg opponent. Fully expect to cancel your expansion Hatchery and to transition away from the 15-Hatch Opening when that attack occurs. If Banelings are detected, it is best to match your opponent’s Banelings with Banelings of your own. 

Scouting
The most important aspect of the 15 Hatch Opening is your ability to scout and obtaining information about your opponent’s build and unit choices. Knowing the size your enemy’s army will help determine how many defensive units you will need to defend against an attack, the repercussions of under or overestimating your enemy’s army are severe.

Transitions
The versatility of the 15 Hatch Opening allows you to transition into any counter based on your opponent’s unit composition. 

The most common Zerg opening, the 15 Hatch Opening focuses on the Zerg’s greatest strength, macro game play. Scouting and an appropriate response to your scouting information ensures you will have a strong upper-hand as you move from the early game to the mid and late-game.