PORTADOWN slipped further behind in the title race following this draw at The Oval.
Andrew Waterworth squandered an 11th minute opportunity to open the scoring for the Glens.
The former Hamilton Academical man was upended in the area by Brian Gartland but after dusting himself down to take the subsequent spot-kick he failed to beat Ports 'keeper David Miskelly who successfully dived in the right direction to save.
But Waterworth redeemed himself on 23 minutes when, after connecting with a Stephen Carson left-wing cross, he powered a left-foot drive home in off the far post.
However, the Ports levelled just three minutes later through substitute Conor McCloskey, on for Sean Mackle, who beat Elliott Morris with a low drive from just outside the area.
And the visitors got their noses in front on 63 minutes in somewhat fortunate circumstances when a Neill McCafferty effort was knocked out by Morris only to spin back off Colin Nixon and into the net.
Glentoran salvaged a point, though, a minute into stoppage time after Darren Boyce volleyed into the top corner after a Carson cross was only partially cleared.
Glentoran: Morris, Nixon, Clarke, Ward, Waterworth, Boyce, Carson, Hill, Callacher, O'Hanlon, Greacen subs O'Kane (not used), Martyn (replaced O'Hanlon 54), Gibson (not used), Murray (replaced Hill 85), Burrows (replaced Waterworth 85)
Portadown: Miskelly, Mackle, Redman, O'Hara, Breen, Tipton, Braniff, Tomelty, McCabe, Gartland, McCafferty subs Murphy (not used), McCloskey (replaced Mackle 14), Paget (not used), Ramsey (replaced McCloskey 89), Clarke (replaced Tipton 29)
- Kevin Braniff and Colin Nixon step out at The Oval (Picture: Russell Pritchard/PressEye.com)