The 20-year-old was surprisingly released by the Welsh side at the end of last season and is currently on trial with the Saddlers.
Marx, who played for Walsall in their pre-season friendly at Rushall on Friday, featured 10 times for Keates while he was in charge of Wrexham.
Now a host of clubs are chasing the promising centre-back, with Keates interested in taking him to the Banks’s Stadium and then loaning him out next year.
“Olly is a strange one – he’s been allowed to leave Wrexham and is somebody I am looking at and somebody I am interested in,” the boss said. “It’s a deal we might get done but I know there is a lot of interest in him from elsewhere as well. “He is young. In my time in charge of Wrexham he played 10 games for me. He had two loan spells out, including one at Rushall.
“He’s not someone who I’d say will come in and go straight into the first-team. But if we get a deal done we will probably look at getting him out on loan.”
Alongside Marx, Keates is believed to be on the hunt for two other centre-backs ahead of the new season.
But while he is closing in on one permanent arrival, he did miss out on loan signing last week.
“There are one or two irons in fire,” the boss continued.
“It’s just a waiting game. There are a lot of clubs hanging fire at the minute on their younger players.
“They won’t let them out because they have got players at the World Cup and there is a possibility they could get injured out there.
“There are one or two we have missed out on but that happens.
“One of the centre-backs we were looking at was a loan but he has gone elsewhere.
“But there is another one I am looking at on a permanent and if I can pull that one off I will be delighted.”