Ok, and in what account modification is this GL defined? Like BSX??? That should be the inventory stock balance account. You can not have OIM on material balance GL.
Stock in transit should be solved not using pure FI. Lot more sophisticated is usage of real stock in transit in you logistic structure ( together or otherwise with new delivery document type etc. ). Where you confirm and book it out form you balance later after you have confirmation form your customer he received it .
From vendor point of view it is similar. But bit more complicated as you will need post GR for PO or SA after you have confirmation that goods are on the way. This postings will be also into your virtual stock in transit.
For both you will need to create new general modification in OBYC transaction GBB. Here you can assign stock in transit GLA.
This modification have to be than linked to new material movement which using OMWN.
So to summarize it, at the end you will have:
From logistic point of view:
1) new virtual stock for material in transit
2) new material movement which have to be used
From FI view
1) new general modification in OBYC/GBB with new GL for stock in transit.
Than you will be able to check accounting value direct by checking your GLA and what is here lot more better, you can check what materials are in transit directly by display you "material in transit" virtual stock. That will give you even more details than just check OPs on GLA.
Jan