You can use the new "return a client" endpoint that was just released. It includes the OS, first & last seen times, among other helpful details.
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GET /networks/[networkId]/clients/[id_or_mac_or_ip]
Successful HTTP Status: 200
{
"usage": {
"sent": 138.0,
"recv": 61.0
},
"id": "k74272e",
"description": "Miles's phone",
"mdnsName": "Miles's phone",
"dhcpHostname": "MilesPhone",
"mac": "00:11:22:33:44:55",
"ip": "1.2.3.4",
"vlan": "255",
"switchport": null,
"ip6": "",
"firstSeen": 1518365681,
"lastSeen": 1526087474,
"manufacturer": "Apple",
"os": "iOS",
"user": "null",
"ssid": "My SSID",
"wirelessCapabilities": "802.11ac - 2.4 and 5 GHz",
"smInstalled": true,
"recentDeviceMac": "00:11:22:33:44:55",
"clientVpnConnections": [
{
"remoteIp": "1.2.3.4",
"connectedAt": 1522613355,
"disconnectedAt": 1522613360
}
],
"lldp": [
[
"System name",
"Some system name"
],
[
"System description",
"Some system description"
],
[
"Port ID",
"1"
],
[
"Chassis ID",
"00:18:0a:00:00:00"
],
[
"Port description",
"eth0"
],
[
"System capabilities",
"Two-port MAC Relay"
]
],
"cdp": null
}