http://www.sarna.net/wiki/OmniMech
The technology was around back in 2750, so it would be old Star League technology. Kerensky took 80% of the Star League with him during the Exodus. It stands to reason that the technology may have been improved upon by the Clans, while becoming part of LosTech in the Inner Sphere.
"The
Mercury was displayed for the first time in
2742. The 'Mech debuted as a replacement for the aging
Wasps and
Stingers in use by the
SLDF. The
Mercury is built around its LTV 160
fusion engine and the
MASC system, that acts on the 'Mech like a supercharger and allows it to reach speeds of 172 kph for short bursts. The
Mercury also features an advancement in weapons installation, using a modular replacement system to allow easier repairs to the 'Mech. Unfortunately, the
Mercury all but disappeared from the
Inner Sphere after
Operation Exodus, and the unique modular repair system fell into disuse and stagnated. The full potential of modular weapons would not be realized until the appearance of the
Clans and their
OmniMechs.
"
As for the Clan implementation :
"The modular construction allows MechWarriors to customize their machine's weapon loadout to suit their own skills and preference, and suit specific tactical or environmental needs as required."
"Though not solely created for modular weapon loadouts, the OmniMech design greatly aids repair and construction as well."
"The technology also greatly aids salvage, even if an enemy OmniMech is virtually destroyed, an intact OmniPod can be detached and reattached to a functional Omni just as quickly and easily as it was factory fresh, maximizing the Clan's overriding drive to avoid waste."
Basically, Omni technology helped the Clans become more effective in not only battles, but a full campaign as well.