Shaq says he gave Oscar De La Hoya a pass when the two men fought back in 2009 ... telling TMZ Sports he "held back" on ODLH because he didn't REALLY want to beat up his friend.

Oscar & Diesel squared off for his old TV show, "Shaq Vs." where the big fella would go up against other pro athletes in their different sports ... with awesome results.

De La Hoya won via decision ... but when we got O'Neal out in NYC he told our guy he basically took a dive.

"I held back 'cause he was my friend."

Not everybody got the homie treatment though ... Shaq told us he had another fight, this time with boxing great Sugar Shane Mosley where he "turned it up a little."

We found the video -- and yeah, Shaq hit Mosley with some REAL power shots and bullied him around the ring like a rag doll.

Mosley ended up winning that fight in a 5-round decision (and hit Shaq with some power shots of his own) ... but it's clear Shaq was trying to send a message that he's no joke when it comes to fighting.