Not balanced compared to other beast companion options
Let's start with the baseline. The Beast Master is limited to choosing a companion that is (PHB, 93):
...a beast that is no larger than Medium and that has a challenge rating of 1/4 or lower.
We can look at Beasts that are a CR of 1/4 or less and compare against the build you have here.
First off, ...
Your baby Owlbear is mostly balanced save for a few considerations.
The size could be small and not tiny, given that owlbear adults are large, and for the claw attacks to be as effective as they are described in the stat block.
For comparison it is best to consider the “Beast of the Earth” stat block from the recently published Unearthed Arcana: Class ...
For me the main problem is the interaction between spells and the beast, it is really cool to have shared spells at the capstone but will be great to be able to use spells like zephir strike in your best companion and not you, as it was previously stated the lack of synergy between some spells like hunter´s mark is not optimal.
One common idea in which I ...
Animal Companions are Minions, and have different rules about how they act. The Druid's Mature Animal Companion Feat alters this, but not significantly.
Haste grants the Quickened Condition with the restriction that the extra action can be used as a Strike or Stride.
Minions don't have a turn and will never benefit from Quickened.