New Build - Bowed Lumber & Holes In Sheathing
You've asked about two distinct things here. I'll address them both, but in the future please ask just one question per post. 

First, bowing in lumber is unavoidable. It's a natural product, after all. The question is one of deflection tolerance. My family's company wouldn't use anything that was bowed on the strength axis more than about 1/4" in 8 feet. Even with good culling, it's critical to "crown" all lumber--align the bows to the same direction--to minimize variance from one stud or joist to the next. 

Bows on the thickness axis can and should be straightened to a reasonable degree when sheathing is installed. They don't have an impact on strength or rigidity, but unaddressed deflection makes everything more difficult, from drywall installation to cabinet setting.

What your builder considers acceptable is at his/her discretion, and is ultimately up to you and your contract. Bring it up for discussion in a respectful manner if you're not satisfied. 

Secondly, gaps in wall sheathing aren't necessarily a concern. Most sheathing specs call for an expansion gap, in fact. If the sheathing has an integrated air/moisture barrier the joints will probably be sealed with specially-made tape anyway. Read the literature for your product and raise any issues with your builder.

As to the holding power of the sheathing for siding fasteners, they'll just need to fasten to studs. Soft sheathings (fiberboard, gypsum) have been used for decades and this is how it's done. 

Finally, OSB is still commonly used for exterior wall and roof sheathing. Why your builder is using a different product is another point for discussion. How does it integrate with the rest of the insulation and air barrier plan? What's the final R-value of your walls going to be? I'd have expected that all that would be in your contract and had been discussed before now, but it's better done late than never.