The 23-year-old has obtained a work permit and agreed a three-year deal and is continuing to train with Pompey.
But the defender needs to come to an agreement with Kaizer Chiefs before making his debut for Pompey.
“As far as he is concerned the contract has run out – but there is an optional year in the contract,” said Pompey chief executive Peter Storrie.
“However, there were no figures along with that, therefore under regulations, that would not be classified.
“It is a legal argument and we are on the outside of it.
“It is between him, his representatives and Kaizer Chiefs and will probably involve the South African FA before eventually having to go to player disputes.
“It is not about money or anything to do with us, it is purely technical and how someone can read that contract.
“If that type of contract was registered in the Premier League it would definitely be null and void and Onismor would be a free transfer.
“The arguments will continue on and on and on.”
