compare uint8_t to char array
When using strcmp(), strings should be terminated with a NULL character.  Which, by the way, is a zero. Your code should work as is if you replace the Line Feed character (10 or 0x0a) with a NULL (0 or 0x00). That is assuming the string you are getting back is 0x4f, 0x4b &amp; 0x0a.  Or you could play it safe and replace strcmp with strncmp.  Then explicitly state the number of characters to compare:

if(strncmp((char *)sigfox_rd_buf,"OK",2) == 0 )