==Formids for Players==
In Oblivion, all objects (a book, a race, a spawn point, a spawned npc) are identified by formids. Formids are eight digit hexadecimal numbers (e.g.0A012C436), where the first two digits represent the source mod and the following six digits are the object index.
While playing, you can usually find the formid of any visible object by opening the console and clicking the object. The object's formid will display at the top of the screen.
Note that there is a difference between the '''object definition''' and a '''placed instance''' of that object. For example the definition ("base object") of the NPC Palonirya (proprietress of "Divine Elegance" in the IC Market District) is 00015ea8, while her placed instance ("reference") is 0002c164. When you click on an object in the game, what you'll see is the formid of the ''placed'' instance (e.g. 0002c164 for Palonirya).
===Construction Set vs. In-Game Formid===
In the construction set (CS), you'll find the formid in the second column of the grid display of most windows. Note that this column is usually collapsed -- i.e. you'll need to widen the column to actually see the formid. For placed references, the easiest way to find the formid is to double click the reference in the cell view window -- the resulting dialog will list both the reference formid and the base object formid.
Note that the modindex may vary between CS and game. In both cases, the modindex represents the order of the source mod in the current context. E.g. the boots from Knights of the Nine is 01000ecf in the construction set, but in game might be 0A000ecf, if KOTN is the 10th mod loaded after Oblivion.esm.
Note too that objects that specifically belong to the savegame and not to any mod are given the modindex FF. You'll actually find these quite commonly: all spawned npcs and creatures, plus all items initially acquired out of inventories and containers, plus all items created in game (spells, potions, enchanted armor and weapons) will all have modindex FF.
===CS Formid to In-Game Formid===
If you know the CS formid and need to know the in-game formid, then you need to replace the CS modindex with the in-game modindex. In other words, you just need to find the modindex of the source mod (e.g. Knights.esp), while you're playing. Here's how:
* Find an object that you know was placed by the source mod. E.g. for KOTN, both the prophet and the Priory of the Nine are placed by the mod, so if you console-click on either of them, then the first two digits of their formid is the modindex.
* If you're running Oblivion Mod Manager, the modindex will be shown when you mouse-over the mod in the main mod listing. (Mistakenly labeled as "formid" in the popup info box.)
* If you're running Wrye Bash, the modindex for all active mods is listed in the second column of the Mods tab.
* '''Oblivion.esm is always first''' (has modindex 00) in both the CS and in-game. No conversion is necessary.
* If a mod has multiple masters (e.g. Oblivion.esm, Cobl Main.esm), then objects coming from the mod itself will have their modindex shifted upwards in the CS, '''however''' the techniques above will still work with no change. (This complexity is something that only toolmakers and advanced modders need to worry about.)
''For folks who don't know what a hexadecimal number is...'' Okay, decimal numbers go like this: 0 1 2 3 4 5 6 7 8 9 10 11..., while hexadecimal numbers go like this: 0 1 2 3 4 5 6 7 8 9 A B C D E F 10 11 .. 19 1A 1B ... etc. Hex 'A' equals decimal 10, hex 'B' equals decimal 11, ... hex 'F' equals decimal 15. Since hexadecimal is base 16 ('hex' = six +'deci' = ten), they "roll over" at 'F' (= 15 decimal), the same way that decimal numbers roll over at 9.
==Formids for Modders==