How to build retro and osr encounters

I see this problem come up a lot from people who are adapting either to a new osr style system, or to a version of Basic, or AD&D. They look at those monster stat blocks and have no idea if it will kill all of their players, or merely inconvenience them. Remember, unless the system specifies otherwise, monsters use 1d8 for their HD.  Here is a handy formula to figure this out:

M=2L 

Your monster threshold is calculated by taking the sum of your player character levels multiplied by 2. This is the highest amount of HD a monster can possess and not TPK the party, at least in mathematical theory. Creatures with spells, powers, and breath weapons, you might need to think  real hard before using, remember, only you as the Referee knows what your pcs can handle.

Now, this is just a guideline and can't apply to every system across the board, for example, DCC is its own system and you may need to exercise judgement, or look at some published material for reference in encounters. The trouble with OSR games is that "balance" isn't a concern, rather the emphasis is on "flavor" "Feel" and "tone" and it is discernible when running a DCC module.

In games that use Basic, or 1stE stat blocks, monsters attack as fighters at the same level as HD indicated and also save as one unless otherwise indicated. Example: a 2HD monster attacks as a 2nd level fighter and saves as such.

2E uses the THAC0 system. Hate it or love it, it's not that bad once you know how to use it. It stands for: "To Hit Armor Class 0". Most 1st level appropriate monsters have THAC0 of 19, or 20. which in modern terms is a +1 bonus for THAC0 19, or a +0 for THAC0 20. Look for a "THAC 0 Chart" and you can find an assortment of nifty tools to speed up play.

The real low level party slayer is right here. Kobolds and wizards beware!