OSR question.
What are the monsters hit bonus or whatever?
Depends on the book.
Originally, there were charts. Then there was THAC0.
I use their HD; so, a 4HD monster has +4 to hit. Works well.
As Erik implies the attack is based off of the HD of the creature. (In rare cases a monster may have some sort of extra bonus.)
The charts in 1e AD&D do have some weirdness in them. Such as a 10 hit die creature can hit someone with an armor class of 0 on a die roll of 10 (THAC0 of 10 -- equivalent of +10 in an ascending AC system). Attack skill increase actually slows down on the chart after that, and any creature with 16+ hit dice has a max THAC0 of 7.
OSRIC - the direct clone I prefer, smooths out the tables. A monster attacks like a fighter as if level=HD. The one note is that a monster with any "plus" to their hit die attacks as one HD higher. So a hobgoblin with a HD of 1+1 attacks like a 2 HD monster, and a Cave Bear with 6+6 HD attacks like a 7 HD monster.
(As an aside you may already know -- HD of 1+1 means each hobgoblin has 1d8+1 hitpoints.)
Fiend Folio is a good classic book if you are able to track it down. There are some I like in the AD&D 1e Monster Manual 2 -- Galeb Duhr, Gibbering Mouther, and Spriggans are ones I recall with fondness.