The club revealed last night that 29-year-old Croatian international midfielder Rapaic will be handed a trial after being released by Fenerbahce.
And the Scottish champions also confirmed this afternoon that Slovakia's former Sunderland defender Varga will join the World Cup star in training this morning.
"We are expecting both of them to be tomorrow morning to join training with the rest of the first-team players," said a Celtic spokesperson.
Both players will not be eligible for the UEFA Cup double-header with Stuttgart.
"There is a long way to go, but we will have a look at them this week and they can have a look at us and I would imagine that they will take in our games against Dundee United and Livingston," explained O'Neill.
O'Neill has also been presented with a welcome dilemma for Thursday's CIS Insurance Cup semi-final with Dundee United after John Hartson returned to training.
The Welshman missed Sunday's 2-0 victory at Partick with a knee injury, but he was hardly missed as Chris Sutton marked his return to the forward line with both goals.
Paul Lambert is also back after being rested while Neil Lennon is expected to stay in the side after an impressive return following a lengthy injury lay-off.