A key question to consider if trying to transfer airside at the International Terminal:
Does OD allow airside transfer?

I can't speak for OD, but I can confirm that other LCCs in the region do not allow airside transfers like this (on separate tickets) in all cases, especially if you require a visa, even with online check-in.

E.g. in Singapore, Jetstar & Tiger do not have transfer desk support. If you require a visa check, even after online check-in, you have to go through immigration & customs and check-in at the landslide check-in.

Suggest you check OD's policy on this.