The 21-year-old, a Republic of Ireland youth international, spent most of the 2019/20 campaign representing the Kent-based side in a prior loan spell, making 32 appearances and scoring once before the season was curtailed due to the Coronavirus pandemic.
O'Connor will now link up with the Gills once again this term, ahead of this weekend's trip to Shrewsbury Town.
Steve Evans's side are currently lying 7th in the table after three games played (W2, L1).
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