Lets ignore the fact that this is Go for a second. The first thing you need is a serialization format to marshal the []string into.

There are many option here. You could build your own or use a library. I am going to assume you don't want to build your own and jump to serialization formats go supports.

In all examples, data is the []string and fp is the file you are reading/writing to. Errors are being ignored, check the returns of functions to handle errors.

Gob

Gob is a go only binary format. It should be relatively space efficient as the number of strings increases.

enc := gob.NewEncoder(fp)
enc.Encode(data)

Reading is also simple

var data []string
dec := gob.NewDecoder(fp)
dec.Decode(&data)

Gob is simple and to the point. However, the format is only readable with other Go code.

Json

Next is json. Json is a format used just about everywhere. This format is just as easy to use.

enc := json.NewEncoder(fp)
enc.Encode(data)

And for reading:

var data []string
dec := json.NewDecoder(fp)
dec.Decode(&data)

XML

XML is another common format. However, it has pretty high overhead and not as easy to use. While you could just do the same you did for gob and json, proper xml requires a root tag. In this case, we are using the root tag "Strings" and each string is wrapped in an "S" tag.

type Strings struct {
    S []string
}

enc := xml.NewEncoder(fp)
enc.Encode(Strings{data})

var x Strings
dec := xml.NewDecoder(fp)
dec.Decode(&x)
data := x.S

CSV

CSV is different from the others. You have two options, use one record with n rows or n records with 1 row. The following example uses n records. It would be boring if I used one record. It would look too much like the others. CSV can ONLY hold strings.

enc := csv.NewWriter(fp)
for _, v := range data {
    enc.Write([]string{v})
}
enc.Flush()

To read:

var err error
var data string
dec := csv.NewReader(fp)
for err == nil {        // reading ends when an error is reached (perhaps io.EOF)
    var s []string

    s, err = dec.Read()
    if len(s) > 0 {
        data = append(data, s[0])
    }
}

Which format you use is a matter of preference. There are many other possible encodings that I have not mentioned. For example, there is an external library called bencode. I don't personally like bencode, but it works. It is the same encoding used by bittorrent metadata files.

If you want to make your own encoding, encoding/binary is a good place to start. That would allow you to make the most compact file possible, but I hardly thing it is worth the effort.

Use a conversion to convert each string to a []byte.

ex := [...]string{"a", "o", ".", ".", "2", ".", ".", "9"}
var ey [len(ex)][]byte
for i := range ex {
    ey[i] = []byte(ex[i])
}

Use this code if your intent is to get a byte array of the joined strings. This code only works when the strings are single ASCII characters.

ex := [...]string{"a", "o", ".", ".", "2", ".", ".", "9"}
var ey [len(ex)]byte
for i := range ex {
    ey[i] = ex[i][0]
}

Use this expression of you want to get a slice of bytes of the joined strings: []byte(strings.Join(ex[:], ""))

I don't know the your context for doing this, but my guess is that it's more appropriate to use a slice than an array:

ex := []string{"a", "o", ".", ".", "2", ".", ".", "9"}
ey := make([][]byte, len(ex))
for i := range ex {
    ey[i] = []byte(ex[i])
}

..

s := []byte(strings.Join(ex, ""))

Safe and simple:

[]byte("Here is a string....")

The easiest method I use to convert byte to string is:

myString := string(myBytes[:])