Frame Overo is the Lethal White gene, if your mare and the stallion are both Frame Overos, you have a 25% chance of a Lethal White foal. LWO - Lethal White Overo Testing
A foal born with LWO has an incomplete digestive system and dies very painfully, usually within the first 72 hrs. I would want both horses tested before I would breed either one of them.
I currently have a frame mare who is in foal to a frame stallion, the breeding was done by the mare's previous owner, and I am waiting to find out if we dodge the bullet or have to put the foal down when it's born. Not a happy waiting situation.
Also, what is your mare's HYPP status? Does she have any lines to Impressive? Are you close to the stallion because I see he doesn't offer shipped semen.