One more addition to CG's list of suggestions:
- Serpents charms will provide a nice boost to mana as well.
FYI, the one thing you'll notice all of these suggestions have in common is that they boost your maximum mana pool. The reason that this is effective is that it the formula to determine how much mana you regenerate per second uses the size of your mana pool. Basically, if you have a larger mana pool, you'll regenerate mana faster.
Wizardspike is a fantastic provider of mana, so by switching to HotO, you've adversely affected your mana for adding +3 skills. IMO, it's a worthwhile swap, because as CG mentioned: