Any body can advise me, i am Danish and rent a condo for 1 year in Pattaya now i like to extend my 90 days stay after 31  july,

for an other 90 days period, i have an O M visa which expire on 20 oktober, i do not know how to do in Thailand ad which papers

i need, somebody can advice me ?

This forum is for those seeking visas for outside of Thailand, I'll move your question to the appropriate forum.

Never heard of a O M Visa.

If you are talking about a Multi Entry Non Imm O Visa you need to do a border hop before end of July. Assuming the borders are open.

 

If you are married to a Thai or are over 50 years old you may have other options depending on your financial circumstances.

 

More info is needed.

I assume you have a multiple entry non-o visa.

If married to or the parent of a Thai you could apply for a 60 day extension of your 90 day entry to visit them.

If not you will have to leave and re-enter the country to get a new 90 day entry on or before July 31st.