Can an Active Army O-5 go to the reserves following separation due to a Board of Inquiry?

I would reach out to the Reserve Component folks on post to see. I had a Major get the ESB screwing just shy of 6 weeks short of 18 years who had bad paper as a LT. He was otherwise qualified and transitioned. He pulled in an AGR job and got picked up for BN Command.

Really don't want to drag out your business here, but if you make it to 18 TIS, you should be in Sanctuary and complete your remaining 24 months and retire. The exceptions to that are usually criminal misconduct, which would likely DQ you from the RC. AR135-100.

If you were relieved for cause, this is a gray area for me. Consult TDS. Really depends on what the GCMCA does. I have seen people get slammed with really severe punishment and been allowed to retire. The other issue here is final grade determination. You retire at the LAST grade you successfully served at. I had a prior service Captain retired at 1LT for a DUI as a Captain.

I would also research the impact of a Show Cause OER. AR623-3.

Having 18 years on the line is a big deal.
I think a trip to the JAG office to discuss your options would be in order at a minimum. And I would strongly suggest you consider paying for a civilian lawyer, preferably ex-JAG, that practices .mil law for a second opinion. It is much easier and generally more successful to fight on the front end of an issue than trying to appeal an unfavorable outcome.

I don't know your situation but I know a LTC in the National Guard who was under inquiry, was released from the Guard. He was allowed to go to Reserves.