Wasak wr wb Simak,
May Allah ease you of your OCD.
Bad or distracting thoughts in salaat are from the shaitan.
Uthman b. Abu al-'As reported that he came to Allah's Messenger (ﷺ)
Allah's Messenger, the Satan intervenes between me and my prayer and
my reciting of the Qur'an and he confounds me. Thereupon Allah's
Messenger (ﷺ) said:, That is (the doing of a) Satan (devil) who is
known as Khinzab, and when you perceive its effect, seek refuge with
Allah from it and spit three times to your left. I did that and Allah
dispelled that from me.
Sahih Muslim 2203
Khushoo is an important part of any prayer.
-Start with concentration during the wudu.
-During salah, think about what you are saying and doing.
-Pondering over the meanings of the Quran ayahs, dhikr and du’as
-Bear in mind that you are conversing with Allah as if you can see Him.
Waswasa affects each person differently, because waswasa has to do with a person’s level of doubts and desires and the degree to which a person is attached or fears other things.
If you continue to get bad thoughts or distractions, seek refuge from shaitan (أعوذ بالله من الشيطان الرجيم ) and dry spit to your left.
Narrated Abu Huraira:
Prophet said, "If anyone of you stands for prayer, he should not spit
in front of him because in prayer he is speaking in private to Allah
and he should not spit on his right as there is an angel, but he can
spit either on his left or under his left foot and bury it (i.e.
Sahih al-Bukhari 416
Also please note that unfortunately lack of concentration during salaats during our times is a very common phenomenon. Your OCD may make it even harder for you but inshallah with explicit concentration and an ever increasing level of imaaan, you will overcome it.
Best of luck!