I had a contrctor come over and said I should not nail my oak floor to my 3/4" Advantech (Chip board) because the nails will eventually come up and the floor mfr will not warranty this. He said you could "probably" get away with it if the installers nailed to each of the floor joists which are 16" OC. Should I be concerned about this? Thanks

Don't use nail/cleats with OSB. Use staples. A 2" staple or cleat is not going to reach the joist. You would need a 3½" fastener to do any good.

Here is a report from Virginia Tech sponsored by NOFMA of subfloor materials and fasteners. It varifies what I have said all along, although others may interpret the findings differently.
https://www.nofma.org/Portals/0/Publica ... Report.pdf

1) Solid board subfloors hold fasteners best.
2) Staples have greater holding power but if tongue breakage occurs, it reduces staples holding power considerably.
3) OSB can perform as well as plywood BUT plywood outperforms OSB IF there are constant cycles of expansion and contraction.
4) This one is surprizing. Manual driven cleats are superior. ?????

There's other info as well. Check it out.