We caught the "Wild Thang" OG outside of LAX, where he opened up about Coolio's death leaving an immense void in his life -- a painful absence he's already felt during his last few performances.

Their friendship runs several decades deep -- Tone tells us Coolio, along with Young MC and the late Shock G, actually performed at his brother's funeral around 16 years ago.

Tone says he and Coolio were regulars on the "I Love The 90s Tour" circuit for the past 6 years, as was Vanilla Ice … who’s also been in mourning since Coolio’s death last week in Los Angeles.

TMZ broke the story -- Coolio died suddenly on September 28 while visiting a friend's home.

At the age of 59, he was still an active hip hopper … with gigs in Vegas and television appearances on "Wild ‘N Out" and "Futurama" already scheduled before he passed.

Tone's DJ M.Walk also chimed in, telling us about his special bond with Coolio ... and it's clear they both miss him dearly, having linked up more than 20 years ago.