Yes, it is certainly the fertile spike of an Ophioglossum. Just to make it clear, Ophioglossum sporangium is not a species or species' name, it means the sporangium of a fern, i.e. the little containers of the spores.
In this photo each of the little knobbles running along the spike is an individual sporangium and when they are ripe they develop a split and shrink a bit so the spores can come out and be dispersed by the wind.
But to identify the species, one needs to see a number of the leaflike sterile blades from which the spikes stick up. Without that no one can guess which species it is except quite often from the detailed s.e.m. images of the spore surfaces.
Did you make any photos of the sterile blades down beneath the spike?
Best wishes,
Chris Fraser Jenkins..