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Ornithine shares the same intestinal transporters as L-Arginine supplementation, and due to that large boluses of ornithine may also cause diarrhea. As this is secondary to the transporter being saturated, the safe upper limit of intake (4-6g rarely causes side-effects) may apply to arginine, ornithine, and other amino acids that share the transporter (L-cysteine) collectively.
The diarrhea is due to these amino acids causing nitric oxide production in the colon, which stimulates water resorption in the colon and osmotic diarrhea. Elsewhere, intravenous or nasogastric infusions of ornithine at 20g or so have been noted to induce diarrhea.
High oral doses of ornithine can plausibly induce diarrhea, but there appears to be a larger safety buffer between the active dose and the diarrhetic dose with ornithine than there is with arginine (with citrulline being free of known intestinal side-effects) *