Charles Oakley is learning the hard way ... if you try to cheat a Vegas casino, you'll end up behind bars. 

TMZ Sports has learned ... the ex-NBA star was arrested at the Cosmopolitan on July 8 after allegedly trying to pull a fast one during a gambling session -- and it was all caught on video. 

Officials from the Nevada Gaming Board tell us Oakley "was suspected of adding to or reducing his wager on a gambling game after the outcome was known."

Unclear what game he was playing at the time -- but one source with knowledge of the incident says Oak tried to pull back a $100 chip after he realized he was going to lose.

FYI, Oakley reportedly made more than $46 mil during his NBA career. 

Oak was confronted about the incident -- and it was kicked over to a security team which reviewed the casino video footage ... confirming the alleged cheating.