Much like Alterac Valley, IOC is not about capping things so much as killing the enemy and getting to their boss. The two bosses are located inside their fortresses which are enclosed in gated walls. (This prevents the "blitz" strategy that works so well in AV.)
Bosses have guards, but these are not particularly hard to kill. A large enough group does not need a formal tank. Just send in the melee and then have ranged open fire. With a few healers working, you should have little problem.
The basic strategy for IOC is simple: control two of the three offensive resources and defend against the third. The resources are: demolishers from the workshop, glaives from the docks, and paratroops from the hangar. Docks also has catapults, which allow your players to fly over the wall into the enemy fort ... from there they can run bombs.
Send your force straight to the workshop. After capping this, continue forward with bombs and demolishers until the seige engines start coming up. Getting the Refinery and Quarry will increase the speed at which seiges appear.
Attacks are often aimed at the west gate. 1) This gate can be easily targeted by glaives, so if you own the Docks, you gain an advantage. 2) This gate is farthest from the enemy rez point inside the fort. However, the east gate is most easily hit with the airship cannons, so if you are getting the Hangar, plan to attack the east.
Demolishers appear soon after the workshop is capped. These mount cannon and rams as weapons. Do not waste time attacking enemy players. Drive the tanks directly to your chosen gate and begin destroying it. (Be sure all your demolishers attack the SAME gate!)
If you put your cursor on the gate, a tooltip will show its state of decay. Its destruction will take a considerable time.
Seige Engine: first appears at 3 minutes and 3 minutes after existing one dies. A seige engine does massive damage to gates, but must be defended. Holds a driver and a gunner.
Bombs: pick up at workshop and drop at enemy gate. A player can carry only one. You can pick up a bomb while mounted.
Capping the docks will give you access to glaive throwers and catapults. As withdemolishers, your target is the enemy west gate ... for the same reasons.
Glaives: these are fragile, but they hit gates hard from long range. You get two glaives, park them just out of range of the enemy fort's tower guns. From there you can destroy the gate in just a few minutes.
A competent enemy will attack your glaives, often by sending stealth. Your team needs to be ready to use crowd control tactics on any enemies who appear. Glaive drivers need to know how to back off and deal with these attacks. Glaives have a long range gun for forts, and a short range gun that is extremely effective against enemy players.
Catapults: also available at the docks. Use these to throw players over the wall. The combination of glaives from the outside and bombs from the inside is extremely effective.
Glaives can operate from out of range of the fort's cannons, but the catapults cannot. Have your ranged players target the west side tower cannons. When the cannons die, the gunners are ejected ... the gunners can then "heal" the cannons. Healing takes about a minute, so be prepared to kill the cannons all over again.
You can cap the hangar, parachute into the fort, and use bombs to break down a gate.
A few minutes after you cap the hangar, two portals will open. These take you to the airship.
Leave a small team behind to defend the hangar flag. Players who die in the enemy fort will rez at the hangar as long as you own it. You may want to have rezzers rotate into guard duty so that no one has to sit there for the whole game.
Put a team on the airship cannons. Cannons have power about equal to a demolisher gun. Use on the west gate of the enemy fort. Be sure to tell the cannoneers which gate to shoot at.
Everyone else parachutes into the enemy fort. Jump early ... as the airship comes up on the first corner of the fort. It looks like you will miss, but the chutes carry a long way.
In the fort divide into two teams. One team runs bombs ... mount, get bomb, take bomb to gate, place bomb, remount ... repeat. Bomb piles are on a timer, so six or eight people are enough for this task. The other team watches the portals and graveyard and kills any enemy players who show up.
UPDATE: this strategy targets the West gate ... because both demos and glaives can easily reach that gate. However, the Game Designers have changed the airship cannon ranges so that they target the East gate easily, but barely reach the West gate. This means that if you have only the WS, you are better off attacking the East gate ... on either fort.
TWO OUT OF THREE
Your goal is to own two out of three of these offensive resources and defend against the third.
For example, suppose you go for the docks and the hangar.
Your plan is to use glaives against the enemy west gate. Glaives will set up just out of range of the enemy tower guns, and you are prepared for enemy counter-attacks by stealth players against the glaives.
You will use catapults to put people inside the fort to run bombs against the same gate.
If your hanger team succeeds, they will jump into the enemy fort, kill the tower gunners, and also run bombs against the west gate.
Since you have left the workshop untouched, you expect enemy demolishers at your west gate. You have gunners in the west tower, and a few defenders ready to attack the tanks. (Most likely your defenders are people who died in the attacks on the docks and hangar and rezzed in your fort.)
This may not be what you started out to do.
Especially in PUG matches, where people on your team may go to all three bases at the start of the match, you cannot be sure which two resources you will acquire. So you need to be familiar with the capabilities of all three.
DEFENDING THE FORT
No matter which strategy you choose, you need a team to defend the fort. A single glaive thrower or a small air assault can easily break the gate of an undefended fort.
Here is a diagram of a fort ... the Allied and Horde forts are the same except for the positions of the grave yards. In both forts the west gate is the one that usually gets attacked.
Gun towers are effective against ground troops and tanks. Leave an extra defender in each tower to guard against intruders who might parachute or catapult into the tower. The guns cannot reach some areas around the gates.
Gates need to be defended with troops on the outside of the fort. Defuse bombs and kill tanks that get into the "dead zones" of the tower guns. Defenders can make use of the teleports to quickly run from one position to the other.
If an air assault is under way, the enemy will be trying to run bombs to a gate. Defenders inside the fort should disrupt this activity, defusing bombs when possible, and killing enemy bombers.
It's possible to defend the boss after the gates open. Often, your forces are at the other end of the map. However, if you have people guarding the Workshop, they may be able to run back to your fort and make a difference. If you are ahead on points, defending the boss may be a better strategy than continuing your own assault.
THE SEIGE ENGINE
The seige engine's flame thrower is especially effective against enemy soldiers. In a stalemated game where neither side can break through to the enemy'sboss, and where the game comes down to reinforcements, the seige engine can winthe game.
For example, suppose the enemy has broken down your gate, and captured the graveyard, but because you have defenders in your fort, your boss is still alive. If you can bring the seige engine to your fort and camp the graveyard, you can easily score many kills.
This means that if the game drags on, you want to make more and more of an effort to own the workshop.
GETTING THE BOSS
The boss in located in a tower at the back of the fort. The tower is guarded by a gate that opens automatically when any of the fort's gates are destroyed.
Once the gate opens, the flag can be capped, which gives you the graveyard inside the fort. This also forces the enemy to rez elsewhere.
The bosses have the same health and level. Neither needs to be tanked.
Once the gates open, your team should zerg the boss as quickly as possible. If you are not up to the task, and if you do not own the keep graveyard, then you will die and rez far away. So do not call "ALL IN" automatically - be ready for it, and be sure that you have enough force on hand.
If you are short-handed, cap the graveyard and call for reinforcements.