Different Christian traditions include different ancient Hebrew texts in the Old Testament, but all agree that at least the Tanakh is included. Other books are added by various traditions and questions about those texts are marginally on-topic and should always be tagged either old-testament or (possibly) apocrypha. Questions that are seeking answers from a Christian perspective should be tagged old-testament. Questions that seek Jewish or neutral answers ought to be tagged tanakh.
If a question asks about just one or two books of the Tanakh, neither tag need be used:
Use tanakh for:
- What is the significance of “east” in the Scriptures?
- According the the Hebrew Scriptures, what happens to people after they die?
Use old-testament for:
- How does Jesus' being from Nazareth fulfill the prophets?
- Do the 12 apostles equate to specific tribes of Israel?
When in doubt, tanakh is preferred since it's more neutral.
For questions specifically about how Christian scripture reinterprets the Tanakh, see nt-use-of-the-ot.
See also: Wikipedia's article on the Old Testament.