Normally I would say that Superman can beat Thanos in a fight. But threads like that sometimes create shit storms. So I'll modify the fight where most believe it's a good fight. Here goes.
Villains usually have the advantage of not holding back and willing to kill.
Heroes, such as Superman, often display mental blocks and hold back out of fear of killing. So in equal bodies and equal ability, villains will have the advantage.
A powerful entity has swapped minds with Thanos and Superman. In other words, Superman is in Thanos body and Thanos is in Superman's body. Both will fight each other. Both have mastered all the powers and abilities of the body they currently reside in. Also, both know what the other is capable of.
Thanos, while in Superman's body, will not be holding back and would use full power to try to kill Superman (who is in Thanos body). On the other hand, Superman, who is in Thanos body, will fight in character. This means he will not fight to kill (begin the fight holding back somewhat) but will still fight extremely smart (he mastered all of Thanos abilities).
And yet Superman in Thanos body still win. It's as simple as this. Thanos in Superman body blitz Superman (Thanos). He feels the first couple of punches and decides to put up a forcefield. Thanos (Superman) is punching away at the shield and Superman (Thanos) starts mind blasting which halts Superman (Thanos) attacks. Superman (Thanos) is trying to fight off this mind attack and Thanos (Superman) decides to pound and blast away at Superman (Thanos) while still mind raping him. Superman (Thanos) is powerful but he isn't tanking hits from Thanos. It ain't happening. Superman (Thanos) gets pound on (while being mind raped) until his body goes limp. Since you're using OWAW Superman, I'm sure I can use Thanos at his best and Thanos at his best withstand hits from High Heralds, shields are powerful enough to withattand an assault from an all out Galactus and blasts are powerful enough to send Galactus himself flying and yes, Galactus durability and strength is more powerful than a thousand earth weights.