If you hold a 1,600 ton master you don’t need the OSV endorsement as it should automatically include a 3,000 ITC endorsmenent. Osv licenses came in 3,000 ITC/500GRT, but according to this checklist it looks like there is a 1,600.
http://www.uscg.mil/nmc/checklists/pdfs/mcp_fm_nmc5_33_web.pdf
(This could very well be out of date, I don’t know how to check. I could CFR it but nah)