The 180° ollie is the natural thing to learn after your basic ollie. I didn't really realise this until recently so I've spent a long time learning to kickflip instead. However, having discovered the 180° ollie I've spent a lot of time practicing this.
The trick seems self-explanatory but is more accurately described if split into frontside and backside 180° ollies. The direction of the 180° turn has such a big effect on how the trick is executed as to make these basically completely separate tricks. A frontside ollie involves turning your front to the direction of travel (and then continuing the spin). For me, as a goofy footed individual, this means turning to the right. You turn the opposite way for a backside ollie.
Unlike the other tricks I have learnt so far, I believe it is best to start learning both of the 180° ollie variations on the move. It is also far easier in my experience to have a small obstacle to ollie up and onto. A kerb is perfect. You will know straight away which direction you feel more comfortable turning in - probably backside. A backside ollie is far, far easier than a frontside ollie.
You may know that I used the nosestall as an intermediary step towards learning to ollie on the move. In the case of a 180° ollie, I used the axle stall...
1. An axle stall is a stall on both trucks. A kerb or other low ledge is perfect to learn this. To begin with, approach the obstacle with your back to it at around 45° with a moderate amount of speed. Your foot positioning should be as per a normal ollie with your back foot slightly more towards the toe edge of the board.
2. Aim to land both trucks on the obstacle. To do this you will have to judge when to pop the ollie. If you are going at much pace at all you'll probably be surprised at how far the ollie will take you.
3. At this stage you'll only need to turn a little to land the axle stall. As you pop the ollie, start to turn your body and push your back foot backwards. This initiates the spin of the board and of you. This movement is similar to how your feet move for a backside shuvit. The difference is that your front foot stays in contact with and controls the skateboard as you and it turn the desired amount (in this case, a mere 45° - you won't need much backwards pop at all).
4. Once you are over the obstacle slam both trucks down on it and stall. If you are using a kerb you won't have much difficulty balancing this, giving you time to compose yourself. When you get bored, either ollie off of the obstacle or simply drop off it by turning away from it on your back trucks.
As you gain more confidence, increase the angle that you approach the obstacle from. Eventually you will be riding up parallel to it facing the obstacle. At this stage you will be attempting to do a 180° backside ollie into an axle stall. You probably won't need to axle stall anymore though so forget that and just try and 180° up and onto the obstacle. Here are some further tips to help you with this, but if you progress from the 45° approach above you will probably have worked this out by yourself anyway.
1. As you roll up to the obstacle wind your body up. You will need to wind up in the opposite direction to the turn so that your front is facing forwards. This isn't strictly necessary but it helps to get the energy required for the turn as your body acts like a store of power which can be released explosively.
2. Unwind and push your back foot back as you ollie. The power of your body unwinding will take your legs with it and will add power to the foot push. The board comes round with you with your front foot again guiding the board.
3. Note that the jump will need to be quite positive to get up and onto the obstacle, but be aware that if you go over the top it is easy to lose control of the board. In these circumstances the front foot, acting as the guiding foot, tends to stay in touch with the board but your back foot loses it. To counteract this, make sure that you yourself are jumping 180° and not just shuving the board with your back foot
4. Depending on your speed you will be landing in switch or in fakie. At moderate speeds you'll tend to land in fakie which I like because it's easier to switch out of, but recently I've been trying it (onto grass) at high speeds and the board is liable to slip just that little bit further landing you in switch. The board is much more likely to fly out from under you when landing if this happens so watch out.
The backside ollie is easier because the movement is less refined. The back push is quite a brutal action, and after doing it your body spin ensures that the board comes with you as long as you are controlling it with the front foot. In contrast, both feet need to control the board well to do a frontside ollie, and the frontside turn always seems much harder to me. The backside ollie is basically closer to a pop shuvit than it is to an ollie, in that you don't really have to level the board out with your front foot - the height comes from the pop as it does with the shuvit.
Again, a kerb or small ledge seems ideal to learn this. I'm not great at these so this trick tip is necessarily vague.
1. Roll along the road parallel to the obstacle with the obstacle behind you and prepare to ollie. You should be in a comfortable ollie stance, with your back foot more towards the heel edge of the board than normal. Wind up backside.
2. As you pop push your back foot forwards and jump frontside, so the pop is more like a scrape. While your back foot is coming up the board will then be well on it's way towards the 90° mark.
3. If you watch someone do a good frontside ollie this is the point at which the board is levelled out. It's a little odd because the foot slide has to be sideways instead of forwards (because the board is at 90° to the direction of travel). As you level out the board start to pull it the rest of the way round with your front foot.
4. Your legs do much more work than the body (in contrast to the backside ollie). The body doesn't really lead like it does when ollying backside, and you will usually find your board has completed the 180° before your body, leaving your top half to turn the rest of the 180° after landing.