Oh Ana

This is definitely about one of the band member's struggles with anorexia. Anorexics personify their disease as "Ana" when they discuss it online forums. If the title was "Oh Anna" I wouldn't bat an eyelash, but because they only used the one "n" it's pretty evident that they're referring to the disease.

"Playing God today" is referring to the contemplation of suicide (slit my wrists and watch that blood evaporate). But if you get to "play God" and take your own life to get out of the horrible spiral of the disease, that can't be good for "Ana's safety" - this is where they're personifying the disease, asking "Ana hear me". They won't kill themselves for the sake of "Ana", their disease and the only 'person' who understands what they're going through.

Referring to Ana as an angel that they couldn't kill is the clincher - having anorexia provides you with a sense of control, a (maladaptive) coping mechanism for things in your life that are beyond your control. Being unable to kill Ana refers to failing to recover from anorexia (perhaps even after interventions and therapy).

There's a Silverchair song entitled "Ana's Song (Open Fire)" which describes its lead singer's struggle with an eating disorder also.