I can not find any definitive answer:
My situation is I am installing 3/4 HW over 3/4 inch osb. I am leaning towards staples, but I keep going back and forth between 2 inch and 1 3/4 inch staples. There are so many conflicting opinions. 2 inch will penetrate, while 1 3/4 will not. It is recommended 2 inch but going through OSB may reduce holding power, yet 1 3/4 inch will not have as much holding power. This is based on the reading I have done. Do you see my problem?! Adding ply is not an option in this case for many reasons. This is what I have to work with. Any help will be appreciated more than you know...

What do the manufacturer's installation instructions recommend?

I purchased from a local manufacture - not a box store or brand. No specifications given...just opinions

Are cleats not an option? Staples are more likely to split the floor board tongues. Staples are less expensive than cleats. Everybody has a different preference. Either length is acceptable....that's why you're getting opinions and not hard and fast rules.

Thank you for your help. Cleats are certainly an option. I was thinking the staple may hold better being OSB as the subfloor.

I have been using staples for years. I have never had a split tongue. Maybe it is my MIII stapler. Staples hold better for me, but still let the floor move when I purposely leave a small gap between rows when the flooring is slightly too dry, so it can finish acclimating after it is laid. As for the length of fastener I don't think it matters much. I have seen cleats rust when they have penetrated the sub-floor. Staples are usually made with galvanized wire so this resists rust, where cleats are bright steel. Special stainless steel cleats are available and can be used for a porch
The important thing is to follow the required fastening schedule. Minimum fasteners per board is two. A fastener within 2-3 inches from each end of the board. A fastener every 10-12 inches along the board.