Interesting, that as a Jarso yourself and not having met any of them. Especially as they are connected with other Oromos on the other side of the former borders. Even we do meet/interact with them, which is how we learn Afaan Oromo.
The history between Geri and Jarso is documented. Those Jarso who lived with us became Somalized either late 18th century or beginning of 19th century. Most likely the former. The first person to do a somewhat documentation of the the relationship and history between Geri and Jarso, was Sir Richard Burton in his book "First Footsteps in East Africa" from 1850s. I suggest that you read it, as I found our relation to be very intriguing.
I also posted an excerpt from another book in this thread. These Jarsos are the ones who've "historically" for the last 200-ish years spoken Somali as native tongue. As far identity, they either claimed 1.Somali, 2.Somali of Oromo origin, 3. Somalized Oromos, or simply 4. Somali-Oromo. As far as clan (don't have much knowledge of Jarso intra-clan affairs), but they've been counted both as part of Somali and Oromo clan society.
Also you're right. Jarso is a large clan, that includes these Somali speaker and a larger Oromo speakers, living Hararghe and adjacent areas (excluding Cinaksen until recently if I'm correct). Though most of the Somalized Jarso has also in recent history spoken solely Somali as mother tongue. Oromo speakers has only increased recently, I'd say beginning in 60-70s. In fact, while majority of Jarso in Somali land speak Oromo today, there are some who still don't. The Somalized Jarso even use Somali clan titles like Ughaaz/Ugaas etc.
With that said, even though the Somalized Jarso are numerically smaller than the Oromo speaking Jarso, the former are the ones we live with, and I have knowledge of.