I'm not talking about the Gaiden one, but the one before it: Beneath Cherry Blossoms in Full Moon.
No it definitely takes place sooner. Because:
- Huang is still alive who died before the end of the first season.
- Mao is still there and in his cat body.
- The inspector doesn't know about Hei's identity. She finds out about it in the OVA but they forget it again because of that potion. But it is hinted in the last episode hat she did have an idea who the black reaper was even though she hadn't seen his face. It could have been that her memories of the events that happened in the OVA returned.