Your /etc/wireguard/[name of your wireguard].conf should be looking something like:
# Input from outside, mind the ingress-rule from Oracle-cloud!
# In my case the Wireguard-port is 59435
PostUp = iptables -I INPUT -p udp --dport 59435 -j ACCEPT
PostDown = iptables -D INPUT -p udp --dport 59435 -j ACCEPT
# Forwarding bidirectional, %i is substituted for the name of the wireguard-adapter
PostUp = iptables -I FORWARD -o %i -j ACCEPT
PostUp = iptables -I FORWARD -i %i -j ACCEPT
PostUp = ip6tables -I FORWARD -i %i -j ACCEPT
PostDown = iptables -D FORWARD -o %i -j ACCEPT
PostDown = iptables -D FORWARD -i %i -j ACCEPT
PostDown = ip6tables -D FORWARD -i %i -j ACCEPT
# In my case the ethernet adapter is enp0s6, change according to your situation
PostUp = iptables -t nat -A POSTROUTING -o enp0s6 -j MASQUERADE
PostUp = ip6tables -t nat -A POSTROUTING -o enp0s6 -j MASQUERADE
PostDown = iptables -t nat -D POSTROUTING -o enp0s6 -j MASQUERADE
PostDown = ip6tables -t nat -D POSTROUTING -o enp0s6 -j MASQUERADE
# STORJ, in my case it's the client with IP 10.66.66.2.
PostUp = iptables -I INPUT -p udp --dport 28967 -j ACCEPT
PostUp = iptables -I INPUT -p tcp --dport 28967 -j ACCEPT
PostUp = iptables -t nat -A PREROUTING -p udp --dport 28967 -j DNAT --to-destination 10.66.66.2
PostUp = iptables -t nat -A PREROUTING -p tcp --dport 28967 -j DNAT --to-destination 10.66.66.2
PostDown = iptables -D INPUT -p udp --dport 28967 -j ACCEPT
PostDown = iptables -D INPUT -p tcp --dport 28967 -j ACCEPT
PostDown = iptables -t nat -D PREROUTING -p udp --dport 28967 -j DNAT --to-destination 10.66.66.2
PostDown = iptables -t nat -D PREROUTING -p tcp --dport 28967 -j DNAT --to-destination 10.66.66.2
Also make sure the firewall from Oracle has been set up correctly. I would set 22/tcp, 28967/tcp+udp and the port of your Wireguard (in my case 59435/udp) open for ingress and stateless in case of tcp. And all outbound traffic allowed for egress.
In the end it can be enabled by: systemctl enable wg-quick@[name of your wireguard]