And that is correct. You can only apply ONE alternative cost to any spell. If you use Omniscience's "free" cost, you cannot use the cleave alternative.

118.9a Only one alternative cost can be applied to any one spell as it’s being cast. The controller of
the spell announces their intentions to pay that cost as described in rule 601.2b.

702.148a Cleave is a keyword that represents two static abilities that function while a spell with
cleave is on the stack. “Cleave [cost]” means “You may cast this spell by paying [cost] rather
than paying its mana cost” and “If this spell’s cleave cost was paid, change its text by removing
all text found within square brackets in the spell’s rules text.” Casting a spell for its cleave cost
follows the rules for paying alternative costs in rules 601.2b and 601.2f–h.