Is [name_m]Otto[/name_m] as a nickname for [name_m]Oliver[/name_m] too much of a stretch?
(We couldn’t call him [name_u]Ollie[/name_u] because our dog’s name is [name_f]Molly[/name_f])
[name_f]TIA[/name_f]! 
I think if the middle name had a “t” in it, it would be totally fine. Otherwise, I’d say it would be a stretch, but not unusable (nicknames don’t have to make perfect sense imo).
I think it’s fine! My grandma’s name is [name_u]Julie[/name_u] but everyone calls her [name_f]Winnie[/name_f] (there’s a long story behind that), but no one has ever really thought twice about it. I don’t think nicknames necessarily have to have a super clear connection, but [name_m]Oliver[/name_m] and [name_m]Otto[/name_m] aren’t that far off.
TL;DR I think it’s perfectly fine!
I think it’s a stretch. Nicknames can be whatever you want. But I wouldn’t call this a logical or obvious option. I like both names, but if I wanted to call my son [name_m]Otto[/name_m] I’d just name him [name_m]Otto[/name_m]. It’s a great name and doesn’t need a longer form.
I think it would make a lot more sense if he had a T middle name. Something like [name_m]Oliver[/name_m] [name_m]Thomas[/name_m] nn [name_m]Otto[/name_m] would be great 
This is what I think too. Or name him [name_m]Oliver[/name_m] and just call him that, I don’t think it needs a nickname.
Yes, it is.
I don’t think it really works.