Odic wave form was the residual Odic energy buried in a well near Tranchard's Fell in the 1650s. It consisted of Agnes Bates's own Odic potential and the essences of other individuals accumulated in the WitchStar. The Eighth Doctor separated this energy from Agnes Bates in her witch form by modifying a Lokic containment web. This was the only way to restore Agnes to her human form.
The Witch was kept in the well for 350 years. When she emerged from her confinement, she still had violent tendencies of Agnes-turned-witch. She also had some of her memories as she recognised the Doctor. Shortly before dissipating in Mary Shelley's hands, she took the form of Agnes Bates. (AUDIO: The Witch from the Well)