## Combine the split files into a single file named 'OLtrain.db.gz'


```python
import gzip
import os
import shutil

def combine_files(input_prefix, num_parts, output_file):
    with gzip.open(output_file, 'wb') as f_out:
        for i in range(1, num_parts + 1):
            part_file = f"{input_prefix}_part{i}.gz"
            with gzip.open(part_file, 'rb') as f_in:
                shutil.copyfileobj(f_in, f_out)
                
            os.remove(part_file)


combine_files('OLtrain', 5, 'OLtrain.db.gz')
```