Here's a list, starting with the longest and working our way down:
7 Letters:
* cooperation
6 Letters:
* operate
* co-op
5 Letters:
* ration
* acorn
* opera
* peace
* cope
4 Letters:
* core
* cope
* rate
* rope
* race
* pear
* care
* open
* upon
* cone
* tear
* coon
3 Letters:
* era
* oar
* ape
* ion
* ten
* on
* cop
* ran
* one
* too
* see
* per
* toe
2 Letters:
* or
* an
* in
* up
* at
* no
* on
* to
1 Letter:
* o
* e
* p
Total: 38 words
Let me know if you'd like to try this with another word!