I am trying to find evidence for the activation of the theta network in a human fMRI study...

Thanks for your question. Let me draw your attention to the Atlas of the Human Brainstem by George Paxinos and Xu-Feng Huang published in 1995 by Academic Press. The sections in that atlas are cut in a plane perpendicular to the long axis of the medulla and pons. Paxinos and Huang use the term "pontine reticular nucleus, oral part" and abbreviate it PnO. This nucleus is located on Figures 37-50 in their atlas.

At the isthmus level of the pons, some 28 mm above the obex, the pontine reticular nucleus, oral part (PnO) makes its appearance lateral to the paramedian raphé nucleus. It has an overall length of about 14 mm and extends into the most rostral levels of the pons. At that point, three other pontine reticular nuclei join it.

I hope this helps you get started on trying to define the location based on proximal landmarks.