Yep this makes games with Oak extremely dull, as once he reachs lvl7 he is pretty much unkillable. He can get into any fight, and time his shield so when he gets low hp he can just shield, attack one last more time and tele out.
In cataract it's even worse since all he has to do is walk happily past double towers towards a portal:
-if noone notices, just cap the portal, lock and tele/get out,
-if anyone notices and comes/teles in, just fight for a while, shield and tele out.
No other demigod has this kind of impunity, or is able to do anything similar until very late game (lvl7 is fairly early mind you). Any other demigod faces almost certain death if he dares to capture a portal alone and enemy team notices and comes to prevent it.
I used to think shield extremely long cast delay (35 secs) sorta balanced things out, but it's getting highly abused lately.
Inmunity to stuns could be delayed until lvl4 shield, or maybe increase the shield cast time 0.5, that way you'd have a small chance to stun him before shield came, but that would probably change the whole shield mechanics too much.