uint8_t *data; // Have your data in this buffer
size_t data_len; // with this length
struct opm_file opm;
int r = opm_file_load(&opm, data, data_len);
if(r != OPM_FILE_SUCCESS) {
	fprintf(stderr, "Could not load file: %s (%d)\n", opm_file_error_string(r), r);
	exit(1);
}

size_t write_fn(void *buf, size_t bufsize, void *data_ptr) {
	FILE *f = (FILE *)data_ptr;
	return fwrite(buf, 1, bufsize, f);
}

struct opm_file opm;
opm_file_init(&opm);

FILE *f = fopen("out.opm", "w");
int opm_file_save(&opm, write_fn, f);
fclose(f);