Getting a better conversion has to do with were you put the OTO in the process. Everyone will say "Put the OTO immediately after the signup" others will say "Put the OTO after they have confirmed their email address".
What I say is "They are both wrong!"
OTO's are useful and very powerful... if used correctly. Trouble is, in my opinion everyone is using them in the wrong places. (When I say everyone I mean everyone who isn't as well known as Mike Filsaime or Frank Kern)
Here's my school of thought.
1. To get the greatest response to an offer they (your subscribers) must know - like and trust you. That is a given and everyone knows this.
2. When a person subscribes to your list for the first time they DO NOT know, or trust you... They may like you but that's besides the point.
3. Instead of putting a OTO after someone first subscribes to your list that will result in low conversions 9 times out of 10... why not use that valuable space for a quick 4 question survey?
This survey will ask their biggest problem and how they best like to learn (Audio, video, written report, etc...)
This information can be used to bring your list members what they want, solve their specific problem and create a product they will buy and present it the way they like.
Personally I only use OTO's to my existing list, when I release a new product or give them a gift. After they make their purchase or download the gift then I present a OTO... because they already know - like and trust me.
I find my conversions are much better when using OTO's in this manner. Not to mention all the valuable information I gather at the survey when they first subscribe.