Johnstone, who played for the Gers in the 1970s and 1980s and is a club legend, rates O’Halloran highly.
However, the former Scotland international has suggested that he is not the type of striker that Rangers need at the moment.
“I would like to see a bigger striker, someone who will give us an alternative approach and be more of a focal point up front,” Johnstone has written in The Glasgow Evening Times.
“That is obviously not O’Halloran’s game but he would be another really good option across the front three and it would be a great deal to get done if we can clinch it in January.
“He has been one of the stand-out players in the Premiership this season and the fans would have been impressed with him when we played St Johnstone at Ibrox in the League Cup as well.”
O’Halloran has been on the books of St. Johnstone since January 2014 and has a contract until June 2017.
The Saints, who finished fourth in the Scottish Premiership table last season, are doing relative well this campaign.
Tommy Wright’s side are fourth in the standings at the moment with 32 points from 21 matches.
Rangers are at the top of the Scottish Championship table at the moment with 50 points from 20 matches.
Mark Warburton’s team are five points clear of second-placed Falkirk and also have a game in hand.