We had some discussion and made announcements in game the other night regarding obliviate. Here are our guidelines for the wand spell:

Students are not capable of doing anything dramatic as far as obliviate goes. Even adults are not typically as skilled with the spell as Lockhart was in Chamber of Secrets. Even Lockhart was not able to replace erased memories with new ones.

Please bear those things in mind while roleplaying the use of obliviate. And also remember that that's a harsh thing to do to another person, and certainly wouldn't be favorably looked upon by professors or other students.

Obliviation by a student might remove a specific memory immediately in the past, or make the last few minutes or couple hours a bit hazy. Feel free to roleplay it in more specific terms as long as both parties agree, but remember that there's only one Lockhart.