OD is trash heres why:

1) you cant build mask of madness on him for a cheap attack speed upgrade (600gold MOM vs
4800+2500gold drow vs 4800gold venge)
2) gets outclassed by almost every dps towers because of this limitation alone
3) scales poorly with items (hard to build attack speed and its item trident is expensive)
4) mana regen is too hard to scale relative to physical damage and magic damage heroes (hero synergy is too weak with almost everyone else)
5) Q is useless because of high mana cost and slow cast animation and the slow nerfs and aghanims is even more useless on him because you are not putting OD besides caster heroes
6) at the current patch you need this hero Early fully online (before wave 100) as the ogre frostmage + wildwing + mud golem comb at wave 100 is impossible to kill as they collectively have 100% physical damage reduction on wildwing 100% magic damage reduction on mud golem at the same time together with their legendary buffs


My Suggestions
1) make his Q crit on his pure damage that scales with his mana regen to make his Q useful
2) add a small splash radius on his W (50 radius wont hurt)
3) aghanims upgrade: makes his base pure damage scale based on his base damage too


Mirana Suggestion
make her Aghanims upgrade on her Q pierce magic immunity and reduce magic armor instead because it defeats her purpose as a magic amplifier tower with aghanims when magic reduction of mud golem goes 100% at wave 100

Queen of Pain
make her Aghanims upgrade on her q fear enemies to reverse direction slowed instead of mana regen