I was playing SF4 the other day and was just amazed at how many and how prevalent OS is in that game, and was thinking why don't we talk about MB OS more. Most of the characters (well, maybe a couple) have their own individual ones, but I'm going to cover a couple that works with a large part of the cast.
OS Bunker Throw
Technically this works with anyone, but it's best and easiest with H moon. You input it like this:
-----5A6EE
On block, it will just do the universal H moon 1-2-3 series. But if they bunker, you will just throw it. A practical application of this would be using some move that kept them in blockstun for a long time. For example:
-----5B 5C 5A6EE
Most 5C's tend to have large amounts of blockstun, which makes it a good way to bait bunkers. One thing to keep in mind is that the 3rd hit of the A series is not normal cancellable on block, so if you don't have a special move that is somewhat safe on block, it makes this string a little bit weaker.
OS Shield/Throw Tech
This works with anyone. You input it like this:
-----1A+D
This will shield any A move or low move, or will tech a throw. If they don't throw, it will just look like you did a low shield. This OS is extremely weak against command grab characters.
OS shield counters
This OS is especially good against H moon, since on shield they auto counter, but does work on F moon if they decide to shield counter after a held shield. There are many ways to do this OS, so I'll just give an example:
-----5B [7] 5C 5A
Basically, you do a string that keeps them in blockstun once you start (like the throw bunker OS). However, you hold 7 after the first move. If you get shielded, you will jump and block the move. However, don't think small with this one! If you have a character that has a normal that is cancellable on the last frame (as in, delay the next normal as late as possible), this makes your blockstrings way stronger against shield. For example, KohakuH can do:
-----2B(2) [7] 5C
If you delay the 5C and they shield that, you will still jump and block the shield counter. Any character that has a normal that can be delayed into another normal that late can do this.
OS Backstep
Although this tends to be character specific, most of the cast has an OS that punishes hopping backsteps and sliding backsteps. You input it like this:
-----2A 2C
If they just block, your string starts as 2A 2C. If the 2A whiffs (because they backstep), you get them with 2C and confirm with a combo. Some players like adding an extra move that gets the opponent to the right height to confirm to an easy aerial. For example, with WarcC, a common anti-backstep string would be:
-----2A 2C 5B
On block, you just do 2A 6C 5C, then go into 5A or whatever normal you want, but when 2A whiffs, 2C 5B will catch them and set them up for an easy air combo. Most characters have a blockstring that does something like this, but the inputs can vary. Generally though, you can count on 2A 2C doing a decent job of this, especially if 2C is a sweep.
Mixup Easy Confirm
This is definitely character specific, but I felt like it was worth mentioning. Basically, whatever mixup you do, you create a blockstring after it that starts with your optimal combo starter. For example with WarcC you can do:
-----{mixup}5B 5C
You can delay 5C pretty late, so on hit you do 6C 2B 2C into whatever, and on block you do 5A and continue pressure.