Hello Paul,
I visited today the archive in Vechta and checked the parishes around Lindern for marriages.
I believe, the origin of the Ortmanns is the Zeller(big farmer) Johann gerhard Ortmann in Ermke, parish Molbergen.
I found also this marriages:
oo 09.Sep.1739 Molbergen, Gerd Ortmann - Maria Crone
child: Jan Gerd, born 27.Dec.1741 in Ermke-Molbergen
(of course they have more children)
oo 17.Feb.1787 Molbergen, Gerd Weßel Ortmann - Mar. Christ. Wendelen, gt.(called) Hermes
I could not found your Gerardus and Fr. Sander. It could be, that they married just outside of the Oldenburg province in the parish Werlte, part of the Bistum Osnabrück, this is close to Lindern.
The archive for this parishes is in Osnabrück. May be you can check this by the LSD.
The phone register shows some Sander(s) addresses in Werlte.
I hope it help,
Werner