There are two factors that will mostly determine if OEM crossbars will work for a Roofnest.
The first factor is weight capacity. The bars will need to have a listed weight capacity of at least the weight of the tent which you are trying to install. This information can usually be found in the owner's manual or a quick google search.
The second important factor is clearance underneath the bars. You need to make sure that there is enough room for your hand to fit underneath the crossbars, as this is how you will be tightening up the hardware once the tent is mounted. Some OEM bars (we are looking at you 4Runner...) are so close to the roof it makes it impossible to get your hand underneath them- most OEM bars are OK.
If your OEM bars aren't up to the job, check out this article for more information on how to get a great aftermarket set-up.