Normal Hardwood floor installation on a wood structured deck
The wood outside will expand and contract due to changing humidity.  You need to provide some spacing between the boards for this.  If you're installing on a humid day, the spacing can be narrower than if you're installing on a dry day.  Any wood kept outdoors will experience a much more varied change in humidity than indoors, that's why the installation can't be quite the same.

Put on a finish that has UV protection and water protection.  Make sure it doesn't turn your deck into an ice rink if it's wet.  The other thing to worry about is that if you only finish the top side, the boards likely will cup since the unfinished underside will absorb moisture.