The thief is probably the most discussed class in all of Basic D&D. It has strange percentile mechanics not shared by any other class. It has backstabbing with poorly defined mecahnics. What even is hiding in shadows anyway?
We're going to explain all of this, from the context of B/X D&D or OSE. But we'll need to rewrite some of the underlying system. I'm not making huge breaking changes to any rules, just restating things slightly differently. This all starts with
Surprise is basically D&Ds badly worded stealth mechanic. This is clear from facts such as evasion automatically succeeding for a surprising group. This basically means that surprise isn't the exact moment of surprising your opponents by bursting out in front of them: rather it is being in position to surprise your opponents should you want to, or simply leaving without ever being seen instead.
So lets rename the mechanic, and shift it around a bit. Rather than having monsters and players roll to find out if they are surprised, we'll have them roll to find out if they are surprising, or rather have the oppertunity to surprise.
Encounters, Surprise, Surprise Checks
Each side rolls 1d6 to determine their stealth, lower is better. Most monsters and characters will be surprised by any side that rolls a 2 or lower. Some might be perceptive enough to require a roll of a 1.
If a group has surprises another, they may act with a one round advantage (performing an action surprising to their foe, and then entering initiative as normal).
If a group has stealth, they may evade combat automatically.
So, we've rewritten surprise into stealth, and preserved mechanics for the times when monsters have "only surprised on a roll of a 1" written in their stat block.
In short, you roll your stealth, if your stealth is good enough you "have stealth" and "can surprise". Stealth is passive, surprising is active (taking your surprise round).
Now it's time to rewrite our first part of the thief!
When attacking a surprised opponent from behind, a thief receives a +4 bonus to hit and doubles any damage dealt.
A minor change, we simply changed "unaware" to "surprised". Now, a thief who has stealth gains their back-stab bonus when attacking in their surprise round. I rule that they don't need to actually stab their targer from behind, as that adds pointless facing tracking to miniature combat which I can't be bothered with, but that's a minor change.
This gives a much clearer guidance to when the thief can backstab. At the start of combat, if they have stealth, and choose to surprise the opponent by popping out of stealth and shanking them.
Now that Back-stab is out of the way, it's time to move on to the thief skills proper.
Climb sheer surfaces
Anyone can climb, thiefs can climb sheer surfaces others would not be able to.
Find or remove treasure traps
This one is handily decently explained by BD&D. A thief can use it to find and remove traps on small delicate things such as a lock. Others can all help in the search for room traps, but treasure traps require the delicate and steady hand of a thief.
So there's this fairly odd mechanic called "Listen at door" in OSE. I suggest you treat this as a general "listen" skill that everyone has (1-in-6 for humans, 2-in-6 for demihumans). I talk about this a bit more in my article on skills. The thiefs hear noise skill is an upgrade to this, making them start in par with demi-humans and advance to a far greater level of skill than anyone else.
Hide in shadows
Anyone can hide, we call this stealth, and it gives you surprise. Hide in shadows is the reason we introduced this mechanic. Outside of combat, time either moves in 10 minute increments, or 1 day increments (dungeon or overworld); at these timescales, the tracking of preceise party position is usually not recommended. As such, we have narrative freedom over the positioning of our thief.
This is useful for how I handle hide in shadows, because it allows us to say the thief is always in the stealthiest place possible at the start of an encounter. If the party fails to have stealth, the thief will call for hide in shadows to grant them an additional oppertunity for stealth. In a way, you can think of Hide in shadows as being a saving throw versus being noticed by the enemy when an encounter begins. This gives the thief additional ability to be stealthed, and should they want to take advantage of their backstab ability.
Other than this, hide in shadows can be used during evasion and persuit. If the thief is being chased, and finds somewhere to hide while having broken line of sight, they can use hide in shadows to evade automatically (as they would gain stealth, and stealth enables automatic escape).
Stealth doesn't have to be converted into a surprise round immediately however. If the party and monsters begin engaging in battle, and the thief stays perfectly silent and motionless where they are, they will retain their stealth and can claim their surprise round between any two future rounds. This is useful for situations where the thief might want to wait for the party to engage before taking their surprise round, so that they are not the only person in danger of being attacked by enemy swords if the party should lose initiative after the thief attacks in their surprise round.
Move silently allows the thief to reposition while retaining stealth. They obviously must not run straight across an enemies line of sight, but for simple out-of-the-way repositioning without triggering a surprise round, move-silently is the roll to make.
In addition to this, move-silently acts as a saving throw versus an enemy listen check. Just as characters have a general listen skill, so too do monsters, and whiehver orc is left on watch overnight is likely using it. If the thief is with the rest of their bumbling party, it wont matter, as the group is heard anyway: but if the thief is scouting ahead alone, an additional save versus being heard will greatly help their chances of stealth.
The number one complaint about this skill seems to be that low level thieves aren't actually very good at lockpicking. This seems to be from treating the open lock chacne as the final chance at picking the lock. If you find thieves aren't good at thieving, make your locks worse. There are examples in old BECMI modules of locks being of poor quality and granding a +X% bonus to pick. Simply apply this more to your locks, and the thief will feel much more successful in your game. I personally don't mind the lower lockpicking chance, but I understand why others are bothered by it.
A 20% chance to steal from someones pockets without them noticing is really good and anyone who thinks otherwise has never tried to pick someones pockets. People generally have very good awareness of their personal belongings, especially in pre-smartphone world where large crowds of shoppers are less frequent and distractions are far less abundant. The 60% chance to be noticed does hurt for hte thief, but that's why the thief has to be careful who they steal from. The game is all about gold acquisition, and pickpocketing the wealthy is a very quick way for the thief to acquire that gold.
So, that's all the thief skills, my thoughts on them, and how I run them. I hope you've enjoyed this read. Most people I've spoken to about how I treat hide in shadows and move silently with regards to stealth and surprise in combat. These rules dont replace any out of combat or rulings-over-rules based usages of the abilities of course! Their purpose is simply to give some useful default procedures that make the thief feel more thiefy and successful.