You go deep into neutral territory or even the enemy base and set up a mobile supply and health room. Also, you go on special missions to kill enemy sentries and hidden snipers. If you're good at o-support, you can make every other offensive player on your team last twice as long. This will create an advanced front, where your enemy's offense won't even be able to get out of the base. Then, since few people will be attacking your base, some of your defensive players can turn to offensive. This will increase the power of your advanced front drastically since defensive players are good at holding fronts.
A great example of this phenomenon is when, in the map Well, you build a sentry on top of the enemy's base, and soon the enemy base is surrounded by your teammates.
The Armor Secret
So, if you find an HW with no armor left, you alone can give him 600 effective health by repairing his armor. The best a medic can hope to do alone is double up his health, adding 200 health. But of course, this wears off; armor doesn't wear off. Notice though that if an HW only has 50 health, then only the first 100 armor he has will be useful since after that he will be dead. So, only repair a person's armor if it will actually make a difference.
If you combine forces with a medic, you can keep your teammates alive indefinitely. They just have to be smart enough to come back to your mobile supply room before they die. You should suggest they do this (see the commander section for how to suggest effectively).
How To Heal
Unfortunately, most allies run around like wild chickens such that it is very hard to give them armor. Some even think you are attacking them! But don't give into the temptation of trapping the ally to give them armor. Running behind your allies is hard, but then so is what they are doing. Do it for the team if not for the ally.
Of course, people can use your dispenser to get armor also. But, most people aren't smart enough to do that, even when you've told them to, and, your dispenser is in only one place. Generally, if the respawn delay is large, your teammates will actually begin to care about living.
The Madmax Duo
It usually wastes too much time if you try to explicitly set up a duo. So instead, just take on the role anytime you see a suitable grunt; the grunt doesn't even have to realize what you are doing.
Establishing a Mobile Suppy Room
The key though is to set your sentry up so it is very defensive, very hard to kill. My favorite example of this is in the map Rock2, in the enemy's underground tunnels. I build a sentry in the center of the concrete tunnel and point it at the ladder leading to the shower room. I place it far enough back that MIRV's don't touch it; and, no enemy can shoot it directly until they go down into the tunnel, into full view of my sentry. One second later the enemy is dead, so they don't even try to kill it, which is what I wanted. Note that it helps if the enemy's yard isn't blown. That way, they can't come in behind my sentry and edge attack it.
The result? Knowing that it's slow but shure, soldiers, demos, and HW's start taking my cleared out route into the enemy base and soon we have a horde of 5 or 6 grunts laying seige to the gas area while I run around repairing their armor. The enemy doesn't have enough people to protect the "spawn point", the shower room, because they are spread throughout their base trying to kill the allied grunts that are already there.
THOU SHALT NOT KILL
Remember that as offensive-support your only goal is to help your allies do damage. This is because you do much more for your team as a support, than as an attacker. With just four hits of your wrench, you can give a soldier another life, and one that starts in the middle of the enemy base.
If you avoid confrontations, and keep your armor up by using your dispenser, you can last a very long time: often long enough for a medic to happen by and replenish your health. As an engineer, you will effectively have 400 health if you keep your armor up until you die. However, at any one time, you only have about 150 health unless you refuel your armor during a fight.
REMOVING THE ENEMY'S DEFENSE
Using your EMP grenades, you can kill sentries and snipers that no other weapon can kill indirectly. For example, in Rock2, your enemy will often put a sentry in the main entrance room. They put it way up on the top catwalk, just across from their little supply room. Besides your EMP grenades, the only thing that can kill the sentry is exposing yourself and shooting strait up at the sentry with a shotgun or some other projectile weapon. Of course, there are often enemy grunts waiting there to prevent you from doing just this.
Solution? Stand outside the enemy base. Prime up an EMP, throw it strait up, and time it so that it blows up at the top of its arc. Since EMP's blast through walls, it will hit and destroy the enemy sentry on the inside of the enemy base. You can do the same to kill snipers that are way up there in the alcoves. This strategy works great in 2fort as well. Any enemy sentry or sniper that is on the top floor can be killed from the bottom floor. When I EMP snipers that are on the sniper loft, I throw the EMP below the floor they are on. That way, they don't even see the EMP coming but it still hits them. However, as with the sentry gun's aim-on, remember that your EMP has the greatest lateral range when it is at floor level with the enemy.