Thomas, 29, and Martin, 26, join on free transfers after leaving Birmingham City and Coventry City respectively.
Both players have signed two-year contracts at the club, who finished second in League Two last term.
Ex-Bournemouth striker Thomas worked under U's manager Michael Appleton in spells at Blackpool and Portsmouth.
"Knowing a manager and knowing that he knows me as a player sort of made up my mind," Thomas told BBC Radio Oxford.
Ex-Southampton centre-back Martin added: "It ticked a lot of boxes for me, especially with the way the club is going.
"Looking at Burton Albion last year, it goes to show exactly what a club like this can do and there's no reason why we also cannot go up again."
Appleton made former Blackburn goalkeeper Simon Eastwood his first summer signing on Thursday.
Find all the latest football transfers on our dedicated page.
Oxford United have bolstered their squad with the signings of striker Wes Thomas and defender Aaron Martin following promotion to League One.